To be honest, the best way is simply to rebuild with the XP cd. BUt you should be able to remove the directory that they reside in. IE, Windows XP goes to c:\windows, Windows 2000 goes to C:\winnt and Windows ME goes to C:\win or something like that. if you delete the folder that contains the OS, it's gone, for the most part. You can look in the boot.ini to see what it says for your boot options.
but the best way, is to reinstall XP and just reformat, as there are files and pointers that are going to be all over if they all live on the same drive.
Good luck