My strong suggestion to you is back up all your drivers as you modem may be the only issue after the upgrade. click here and this will do it for you.
I would also strongly advise a clean install rather than an upgrade install. Start the win XP cd in Win 98 follow the wizard then I think its the 3rd screen of the wizard click the downward arrow and alter it from Recommended Install to Full Installation. Also make the file system NTFS
Make sure you have backed up all your data first as you will loose it other wise.