give me abit more.. do you want to create a dual boot.. or just degrading your OS.
2nd one very easy.
1st one: copy you boot.ini file to a safe place. edit you current boot.ini with removing the windows xp tag [.../fastdetect]. shutdown ur machine and boot again with the 2000[prof] cd and install it in a diff partition. after the install edit your boot.ini with the proper direction to win XP