Whether you meant to or not - you have opted for a dual boot system
The XP installer should have asked you whether you wanted to upgrade or a clean install - I suspect you chose the latter?
You say you want to start again with just XP - are you saying you'd be happy to wipe your disk and start from scratch - i.e. what about your existing files?
There are already numerous threads to do this - search the forum, last time I looked it was pinned.
Why not stick with a dual boot?
I think dual boot is better, if you experience problems with one OS, you can switch to the other
The only down side is that they require more work - you need to install your programs twice (caution needed here - best to create a specific folder for your win2k programs and your XP programs, and instruct the installer programs which folder to install into).
May experience some caveats dual booting Win2k/XP, compared to say Win98/2K or XP - but if you're there already I'd explore the benefits of a dual boot scenario.
Lots of resources on Microsoft's sites on dual booting.
Good Luck - which ever way you go
DaveB