Hello and welcome to the forums.
Since your XP computers are members of a workgroup, simple file sharing is turned on by default. You need to turn it off.
Start > Settings > Control Panel > Folder Options > Click the View tab and deselect Use Simple File Sharing (Recommended)
VMRoyal, thank you! That worked! Matt
Yup, my windows xp problem is solved too.
But i get the same problem with my windows vista. Could you tell me how to fix it? Could this problem appears because windows sharing folder too?