I am looking at using the UBCD to reformat my home computer that uses XP Home Edition. I had to reload windows one other time, and found out that Home Edition does not have the Fdisk option.
Windows XP doesn't have an 'fdisk' the way earlier versions do, but it does (or should) present a dialog at some point asking what partition you want to install Windows XP onto. At this point you can delete or change the existing partition(s).
Press <del> or ctrl-D (or something like that; it lists the options on the screen somewhere) then follow the instructions EXACTLY - you're asked to press several very non-obvious keys and just pressing enter will immediately abort the process. Once you've done that you can install on the 'free space', or create a new partition if you don't want to use the whole disk.