There are a number of ways that you could accomplish this, ranging from ftp to cifs, rsync, etc. I believe fanoush has cifs kernel modules on his website which would allow you to use standard windows file sharing to transfer files to/from the n800.

http://fanoush.wz.cz/maemo/

You might also take a look at Orb, which lets you access all your PC's media content remotely on the N800...as long as you have an internet connection. One thing you might want to consider when transfering data to the n800 is that the wireless is going to limit you to a fraction of a MB/second transfer speeds. You may be better off simply sticking a SDHC card in a reader and plugging it into your XP machine, at least for large files.