[/b]dannyboy[/b] could you give us a little more information. Mainly what brand computer and what do you think it is infected with? Do you have the recovery\restore\OEM XP disk?
Have you looked at our self help section? http://www.bleepingcomputer.com/forums/f/55/spyware-and-malware-removal-guides-and-reading-room/
Do you really want to spend the time to safe you current install or do you just want to start over?
If the drives are infected beyond repair yes you need to clean your drive prior to re-installing XP. If you go to this extreme I would recommend a drive "wipe" in stead of a simple "format". Wiping a drive overwrites the information on the drive, while formatting leave the information intact but allows the old information to be written over.
There are many "wipe" programs and of various degrees of destruction. I find that a Simple Wipe is perfect for cleaning a disk for installation. It will overwrite the entire dive one time. If you are attempting to conduct a secure wipe there are several strenghts that you can use DoD (7 passes), Gutman (38 passes), or specify how many time. 38 passes is Black Helicopter paranoid, and frankly a waste of time.