Usual: boot from XP CD, repartition and install.

When the "usual" doesn't work because Windows can't see/read various linux partitions, I just wipe the entire disk with KILLDISK or DARIK'S BOOT AND NUKE.

There are partitioning tools, such as those offered by the HD mfg., that will delete partitions. There are also open source tools like GParted that may succeed in deleting partitions, but not always.

KILLDISK has never failed me when I encounter this problem.