I take it you only have the one iPod?

If you go into the iPod's settings and select 'About', is is formatted for Macintosh? If so, I think this is where your problem lies: a Macintosh-formatted iPod will not work with a Windows machine and vice-versa.

To get it to work with your XP desktop machine you will have to reformat it for Windows which will stop it working with your Mac.

It's best to only keep the iPod linked with one music library (the one on your Mac?) and use that library to store all the music you want to transfer to the iPod.