|Good advice all round, what you can also do is use the XP cd to reload the operating system without formatting the hard drive.|
9.9 times out of 10 it is the systems files which get infected.
If you do a normal clean install of XP by booting from the cd rom, you will follow the normal install procedure right up to the time it asks you is you wish to install windows using the current folder.
If you chose yes all the XP install will do is replace the operating systems files only.
All other information will remain on the hard drive. But you will need to reinstall all the updates patches and programs again.
If the hard rive is large enough it is always advisable to partition the drive.
Keep the important files on the last partition for safety. As XP is NTFS format you can delete the entire C or perhaps D partition, change their size etc. And as long as you don't touch the E partition your important data will still be there.
Hope that is of some additional help to you.
IF IT AINT BROKE, DONT FIX IT