I've done this. It's a pain in the ass. You basically need to set up a shared folder in Windows XP (I don't know about Vista). But you might need to set up a user account in Windows just to interact with the Mac. I did this wirelessly, but I rarely use it. It's much easier to use other methods, especially since the wireless only transfers up to 1MB/second (if I'm lucky).
If you have a big USB drive, you might want to stick with it.
If you really, really want to try it, I used resources such as these....and
a lot of trial and error:
http://macs.about.com/cs/tipstutoria...sx_sharing.htm
http://www.ifelix.co.uk/tech/3001.html
This setup might be good for huge files you don't want to muck around with on smaller USB devices.
If the above methods don't work, try Google searches for keywords like:
networking OS X Windows XP or something. That is how I found the method that worked for me.