You'd probably have to reformat. You might be able to boot to the XP CD and try to reinstall XP over itself but that will be tricky at best. Then there's the question of chipset and other drivers. There is a method detailed in the Anandtech FAQ's for doing something like this. You should take a look around. If nothing else, just borrow a ME or 98 CD from a friend, format the drive and reinstall XP. When it asks you to insert an OS disk (to prove you are eligible for the XP "upgrade") use the borrowed CD. That should work fine.