[TiberiusAD]

This is easier than you think.

.Put everything you want to save on one of the drives. IE C:/ or D:/
.Insert your XP pro disc and boot from disc
It will upload start-up files, and then ask what you want to do
.Select to install Xp pro on your PC.
.It will then ask you where to install and will show your drives.
Simply select the drive you don't want to keep, and delete it... it shows you how
.Now format the extra space into a new partition.

Now you should have only one OS, and a clean drive.