How did you install your Linux distro in the first place? If from a LiveCD, boot with the CD and use the included disk partitioning software (GParted or suchlike) to wipe and reformat to whatever you want.
If you can boot to Konsole, and understand zeroing a disk, you can try this:
[as root]
dd if=/dev/zero of=/dev/hda(b) conv=sync,noerror count=x
where x = number of LBAs
Maxtor = 320173056
WDC = 16514064
cylinders * heads * sectors
Of course you'll need the number of LBA's for your own HDD. Google turned it up for me easily, or you can work it out from your BIOS.
Pray what possessseth thou to want XP back?
