I've been having some trouble with memory sticks/portable hard-drives that have been formatted on Vista not showing in XP, this was a common problem at my school also (some XP machines and some Vista machines...) but google is being completely useless about it xD Any help would be cool.
File system (NTFS) of Vista is not the same as the one in XP. Do like cpemma suggest. Is Windows XP, at SP3 and FULLY updated?`