WinXP works fine with BG2, even though it's not officially supported. The key is in doing a clean install of WinXP, then doing your BG2 install over that. If you upgraded WinXP over Win98, chances are something may have stuffed up. You can try downloading the patch from
www.bioware.com to see if that works. You may need to upgrade your graphics and sound drivers, they're usually the first cause of the problem.
FWIW I'm running WinXP and BG2/ToB and it works brilliantly.