Or you may try a bootable live image as shown here:
http://talk.maemo.org/showthread.php?t=99093

Of course I would recommend using the Ubuntu Iso promoted in the last post
With this one you get EVERYTHING together for flashing a N900. All firmware files, Howtos (just open firefox, they are all preloaded) and flash commands in terminal bash history.

If you burn a DVD from the ISO file then you can use any (old) computer which is able to boot from dvd. No internet connection needed, everything is build into that iso.
If you boot the ISO in a virtual machine then you have to manage to get the USB port routed to that VM.

Cheers,
Ulle