there is no need for cleaning or scanning the disk - since you are going to wipe it.
normally the procedure is simple :
-boor from the Xp install cd - and when it asks what to do with the partitions, Delete the old partition on which XP was installed - then continue the install.
if for a special reason, you want to wipe the disk before installing, i recommend DBAN(also on ubcd) :
http://dban.sourceforge.net/http://www.ultimatebootcd.com/ here a complete guide for installing XP, as well as all other options :
http://www.michaelstevenstech.com/cleanxpinstall.html