XP doesnt work too well with dual boots...lol It also doesnt like other operating systems nor is it good with when upgrading. Downgrading from XP to 2k can also cause chaos. My best choice is format...lol
However, did u try booting into safe mode (F8/F4 key i think it is), or did u try a system restore (those half the time dont work for me, so I just disabled it, but anyway, worth a try...)?
Lemme know what you come up with.