Files should be no problem, programs may be. Some will not work with xp. If you are talking about sharing across a network, same rules apply. If you are installing XP on a older W98 computer, I recommend you ether reformat the drive (NTFS) and fresh install XP or take your current drive and use it as a second drive and install a new drive for XP. I never recommend an upgrade (disk), you are asking for trouble.
