It could be very difficult .... or it could be very easy, depending on how you installed it. Did you install Windows XP onto a different partition on your hard disk or did you install it onto the same partition (e.g. the C drive) that has Windows 2000 on it?
If you installed it onto a separate partition, you could browse to that drive letter in Windows Explorer, right-click it and select the Format option. Note that this will delete ALL files on that partition so make sure you haven't got any other data on it before you wipe it.
If you installed it onto the same partition as Windows 2000, then I don't think you can get rid of every trace but you can at least stop it trying to start it up. You'll need to delete the files from the directory that you installed Windows XP to, assuming that you installed XP into the C:\Windows directory and you already had Windows 2000 installed in the C:\Winnt directory. You would also need to remove the Windows XP entry from the C:\Boot.ini file, which you can open in Notepad (you may need to switch hidden and system files on so that you can see it).
If you don't know how to stop it booting into Windows XP at all (i.e. you don't know how to get into Windows 2000 now), all you need to do is select Windows 2000 from the boot menu if/when it appears.
Let us know if that helps
