The biggest problem here really isn't so much the tablet's inability as it is a general discerning of technologies. For what you're talking about (syncing folders) something like rsync is a much, much better application as it natively supports it's own network protocol/etc meaning you don't need to use network shares/etc. However I've never heard of anyone using rsync on windows... doesn't mean it doesn't exist though, so might be worth some googling.

One thing I would suggest though is a change of paradigm. Your current problem is caused by a bit of backwards logic... you want your XP machine to automatically sync folders whenever your tablet appears on the network; but that doesn't make any sense since XP never knows when the tablet's there and when it's not without a lot of customized programming. Why not instead do the opposite? Have the tablet sync a local folder (SD card, whatever) to the shared folder on the XP machine whenever it associates with the local AP? This is much easier and practical to do! There's no friendly GUI to make it all work, but a few quick and dirty shell scripts it should be possible... Lookup how to mount cifs shares on the tablet (cifs = windows XP shares). Then look for rsync to sync between your mounted folder and a local one. Finally, something like dbus scripts to trigger it all when you connect to your AP.

(Yea okay that's a lot of searching with only the vaguest of directions, but it's a start... I'll try to give some more detailed instructions this evening or later in the week as this is something that I've been toying doing for a completely different application for a while now).

Thanks,
-Rob