-------this will wipe all data on the drive -------
put ya windows xp cd in the dvd drive and reboot, wipe out the partitions at the select installation location section and create new ones, and away you go......
ok this is wierd, i tried booting off the XP CD and nothing happens it just goes into the windows 7 as normal so i then tried to install off the disk and get the message " a newer version of windows is already installed setup cannot complete"
Try this page as this guy is very knowledgeable about instating and uninstalling many types of operating systems. Complete step-by-step instructions are given,.
its giving you that message because theres no os installed on the drive and its trying to boot from it, you need to recheck your bios settings for the dvd/cd drive as it doesnt seem to be loading the xp disc before looking elsewhere,