My N900 is getting slow, well to be exact, it's moving at normal speed I guess but I believe the touch screen is less responsive now, it could be it has many scratches on the screen guard and so it's less responsive now. I'm thinking of flashing but I have win7 on my pc and also if I use BackupMenu will that really clear the clutter off it?

Reflashing isn't going to improve how responsive the actual screen is.
Replace/remove your screen protector, they get worn out after a while.

Screen guard first, if no difference, reflash and you will have a new looking screen too

There's a chance that some processes running in the background caused the clutter, therefore restore via BackupMenu might not help you in your situation.

I don't recall anything that would cause responsive problem but may be others know. It may be helpful if you'd show us a list of packages you've installed such that anyone spots the abnormality would come to help:

up 1.1.0-2 user/utilities 148
3g2g-mode-selection-applet 0.4-2 user/desktop 64
abook-qrcode 0.4 user/office 100
account-plugin-haze 0.7 user/network 140
adblock-plus-1.0 1maemo0-8 user/network 1548
adflashblock-css 1.0.0-4 user/network 116
advanced-clock-plugin 0.12.3 user/desktop 284
advanced-power 0.4.2-4 user/desktop 188
advanced-power-monitor 0.5.3-3 user/utilities 216
an-droid-icons 1.0-2 user/system 852
angrybirds 1.4.2 user/hidden 37300
angrybirds-levelpack1 1.0.0 user/hidden 820
angrybirds-levelpack2 1.0.0 user/hidden 436
angrybirds-levelpack3 1.0.0 user/hidden 564
app-search-widget 0.5 user/desktop 96
appdownloader 0.2-12 user/system 700
ask-ziggy 0.6.2-1 user/network 924
autodisconnect 0.4.7 user/network 240
bash3 3.2-0maemo8 user/utilities 520
batterygraph 0.3.2 user/utilities 356
bigword 1.0.11 user/hidden 54
black-plastic-theme 2.400 user/system 29044
blubbels 1.0-maemo4 user/games 184
blue-sky 1.9 user/desktop 12164
carbonsteel-theme 0.9-5 user/system 11184
classicprint 0.1-5 user/graphics 1092
com-ea-app-sims 20.0.26 user/games 93082
com-gameloft-app-brothersinarms 1.0.9 user/games 63058
com-gameloft-app-driver 1.1.7 user/games 395735
com-gameloft-app-letsgolf 1.0.0 user/games 140826
conboy 1:0.7.0 user/utilities 676
conky 1.6.1-0maemo5-1.1 user/desktop 680
contacts-merger 0.1.3-0 user/utilities 152
conversations-inbox-widget 1.1.1 user/desktop 96
cpufrequi 0.01-2 user/system 264
cssufeatures 2.0-1 user/system 240
custom-operator-name-widget 0.1 user/desktop 100
cutetube 0.9.10-6 user/multimedia 2132
darksky 1.0 user/system 2548
decoders-support 0.4 user/multimedia 84
desktop-cmd-exec 1.10 user/desktop 144
drawing 1.2.0 user/hidden 8579
drnoksnes 1.3.4 user/games 1260
dtg 1.0.153 user/office 8884
easylist 0.3.20 user/utilities 280
extcalllog 0.5 user/network 180
extended-contacts-search 0.4-2 user/utilities 72
eyes-widget 1.0.0 user/desktop 188
facebook-services 0.5.5 user/hidden 52
faenza-n900-icons 0.1.4-2 user/desktop 3124
fapman 0.7.1-1 user/system 864
fapn 0.6-1 user/network 144
fcam-drivers 1.0.6-1 user/multimedia 452
feedservice-plugin-amazon 0.3.5 user/hidden 320
feedservice-plugin-ap 0.2.22 user/hidden 216
feedservice-plugin-fb 0.5.5 user/hidden 264
feedservice-plugin-fb-common 0.5.2 user/hidden 60
feedservice2 0.5.9 user/hidden 212
fennec 1.1 user/hidden 33192
filebox 0.7-2 user/utilities 2048
flashlight-applet 0.4-3 user/desktop 248
fm-boost 0.2-2fremantle1 user/system 120
fmradio 2010.03.20 user/multimedia 2172
foreca-weather-applet 0.9.5 user/desktop 816
frontview900 0.2-2 user/graphics 3943
gltron 0.70final-9maemo9 user/games 2740
gnumeric 1.10.12-2 user/office 19208
headphoned 1.9 user/multimedia 84
healthcheck 1.2.0-3 user/utilities 248
htop 0.8.3-1maemo5 user/utilities 192
jspeed 0.1-9 user/navigation 1040
kernel-power-flasher 2.6.28-maemo42 user/system 240
kernel-power-settings 0.11 user/system 140
leafpad 0.8.17le-0maemo1 user/utilities 944
led-pattern-editor 0.0.4-1 user/system 188
lens-cover-reminder-sp 0.4 user/desktop 124
libqtm-bearer 1:1.0.2-maemo4+0m5 user/libs 300
libqtm-contacts 1:1.0.2-maemo4+0m5 user/libs 1012
libqtm-location 1:1.0.2-maemo4+0m5 user/libs 256
libqtm-messaging 1:1.0.2-maemo4+0m5 user/libs 848
libqtm-multimedia 1:1.0.2-maemo4+0m5 user/libs 1544
libqtm-publishsubscribe 1:1.0.2-maemo4+0m5 user/libs 244
libqtm-sensors 1:1.0.2-maemo4+0m5 user/libs 336
libqtm-serviceframework 1:1.0.2-maemo4+0m5 user/libs 392
libqtm-systeminfo 1:1.0.2-maemo4+0m5 user/libs 420
libqtm-versit 1:1.0.2-maemo4+0m5 user/libs 468
load-applet 0.4.6-5 user/utilities 100
macuco2 0.1.16 user/network 336
maemo-mapper 3.1 user/navigation 1528
mbarcode 1.0.3-1 user/utilities 28
mbarcode-plugin-qrcode 0.0.7-5 user/utilities 164
mbarcode-plugin-webrequest 0.1.15-11 user/utilities 156
mbarcode-plugin-wifi 0.1.0-13 user/utilities 76
media-im-status-updater 0.4 user/multimedia 132
mediabox 2010.11.11 user/multimedia 3808
mirror 2.1-1 user/multimedia 188
monorail 0.4 user/network 280
mp-fremantle-generic-pr 20.2010.36-2 user/system
mplayer 1.0svn20091221-4 user/multimedia 18612
mstardict 0.6-2 user/utilities 588
mydicto 1.0.3 user/multimedia 96
mypaint 0.8.2-2maemo0 user/graphics 10476
n900-fmrx-enabler 1.5-4+0m5 user/libs 92
nhouse 1.0.0 user/hidden 46972
nhouselite 1.0.0 user/hidden 26960
oculo 0.8 user/desktop 412
offscr-anglemeter 1.0.1 user/hidden 972
offscr-coin-toss 1.0.2 user/hidden 1496
offscr-converter 1.0.3 user/hidden 1092
offscr-elements 1.0.1 user/hidden 1024
offscr-level 1.0.2 user/hidden 784
ogg-support 1.0.5 user/multimedia 136
omweather 0.28.12 user/desktop 4104
omweather-icons-contour 0.3.2 user/desktop 736
omweather-weather-com-stations-db 1.0.2 user/desktop 3516
opera-mobile 11.0beta1-9~fremantle0 user/network 14064
osso-abook-plugin-merge-contacts 0.1.5 user/utilities 276
packageview 0.4.20100104-4 user/utilities 264
pidgin 2.7.7-0nix2 user/network 4392
pidgin-maemo-docklet 0.0.3-1 user/network 96
plastic-icon-pack 1.0-2 user/system 680
pleego-icon-pack 1.1-3 user/system 1444
pleego-remix 0.7.2 user/desktop 7220
pleego-remix-icon-pack 0.3 user/system 2352
purple-extra-protocols 2.7.7-0nix1 user/network 180
qcpufreq 0.4.1-1 user/system 308
qtranslate 1.0.2 user/hidden 200
qtwallet 1.4-9maemo1 user/utilities 1020
queen-beecon 1.3.2 user/desktop 564
quick-launch 1.1 user/desktop 248
recorder 0.3.4 user/multimedia 240
recovery-tools 6.11-9-fremantle5 user/utilities 772
rootsh 1.8 user/system 32
shaded-chat-theme 0.0.4-1 user/system 60
sharing-service-facebook 0.5.1 user/hidden 184
sharing-service-twitter 1.0.00.2+0m1 user/hidden 88
sharing-service-youtube 1.0.00.2+0m1 user/hidden 92
simple-brightness-applet 1.1-1maemo0 user/desktop 64
sixad 1.3.0-0maemo2 user/system 424
sleepanalyser 2.1-35 user/utilities 2000
sliderule 1.0.4-1 user/science 176
starhash-enabler 0.2 user/system 16
stellarium-mobile 1.0.3 user/hidden 21908
storageusage 1.1.4 user/utilities 216
swappolube 1.1-6 user/system 244
tar-gnu 1.22-2maemo5 user/system 2360
tecnoballz 0.92-4 user/games 3336
teeworlds 0.5.2-1maemo2 user/games 744
telepathy-msn-pecan 0.1.2 user/network 1084
tennix 1.0-7 user/games 5300
term-app-launcher 0.1-3 user/utilities 24
theme-customizer 0.9-65 user/system 916
threetictac 0.7-2 user/games 308
tracker-cfg 0.3.2-1 user/system 136
ttf-droid 1.01-dfsg0maemo3 user/graphics 4392
tutorial-home-applet 0.6.16+0m5 user/support 1292
tweakflashver 0.4.0-maemo1 user/utilities 212
tweakr 0.0.17-2 user/system 228
ukeyboard 2.4-5 user/system 480
vgba 3.6.8-2 user/games 418
volumecontroldaemon 0.1.2 user/utilities 108
wifi-assistant 1:0.0.3 user/network 200
wifi-signal-applet 0.4 user/desktop 116
wifieye 1.0-3 user/network 256
wifiinfo 0.10.20100104-2 user/utilities 712
witter 0.3.7-2 user/network 2072
wormux 1:11.04.1-1 user/games 2168
xournal 0.4.2.1-1fremantle21 user/office 812
zenbound 1.0.4-1 user/hidden 13

I also opened set extras devel active a couple times, but not for a long while, just to install nitdroid.

It can be the screen guard but can a couple scratches really make me have to press the X a couple times for a window to close in programmes manager view? or make typing this very laggy even when clocked at 1.2ghz nd using swapplube

You should really update the extras packages from time to time as the update may contain bug and performance fix, other than features update.

However, you'd really want to hold back some package until it's more stable, if you don't want to reflash that frequent.

Say I hold the kernel power to v46, as v47 is a big leap since its previous version.

To hold the update of a package, you can:

The exact package name can be found by dpkg --get-selections.

I do update when I'm notified

Have you enabled package category extras-devel at this moment?

In Fast Application Manager yes in the Default App Man nope

I'm not sure about FAM, I use apt-get.

I ask because I see some of your extras packages have not been updated. E.g. Kernel power.