I think I will be right in saying that You need the exact media for 2000 that was installed as all these are a little different and so can hit problems even if all looks all OK. Have You the correct media?

I never ever upgrade a single application – SP’s are OK as we have little choice but OS and Apps is bad news to much can do wrong and often does – is the media for Xp upgrade / Upgrade – full / full

If at all possible fully remove 2000 with the 2000 disk do not hack it and install full clean Xp always the best move over all

jiuk