Hi
Some viruses can be removed with a little work and a format and reinstall isnt always needed, however if this is the path you wish to use simply boot from the XP CD and follow the prompts to install a new copy of XP. when you get to the part about the available partitions choose to remove them all and then recreate them, this will erease all the data on the drive.
Make sure you have backed up your important data and that you have all the drivers you need before proceeding.
Regards