I was going on the presumption of this being done during setup. Authenticate is right, if you have XP freshly installed on the one partition you want to keep, then you can just use XP's Disk Management to delete and create other partitions.
Linux having been on one partition shouldn't cause any problems with you deleting the partitions, as long as they can be seen by XP. If you're going through the setup process and reach the point that it asks you what partition to use or allows you to change partitions, and I think you highlight it and hit D to delete, then press L to verify the deletion, does it return to the list of partitions and still show the partition you just deleted?