From the title, can I assume you just want to wipe the drive and reinstall XP? Don't forget to back up anything you need first - will be gone after reformat.
To do this, just boot from XP install CD (may need to change bios settings to boot from CD as first boot device). Choose new install - it will probably detect your exisiting installation & offer to repair it. You can take this option if you wish (assuming virus is cleaned off machine) - and you may get back a working machine, with all data, apps & settings intact - just loses windows updates. If you really want a clean install - don't take repair option - and you will be presented with partitioning screen. Use the tools to reomve existing partition and create a new one. format ntfs or fat32 as you want - and XP will then reinstall (you'll need key & have to answer a few questions).