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This might help your PC performance so you can avoid formatting. You may also consider re-installing Windows XP. If you are trying to eradicate a virus, this will not remove all the files. The virus may still be present after re-installing. Most brand name manufacturers include a disk that can restore your computer to the state it was the day you purchased it. Some only include a Windows XP disk, and some provide nothing at all. If you have a restore-disk you can save yourself from having to reformat your computer. Once you begin the process you will not be able to access files on your computer or the web. If you have a printed copy of the necessary steps and troubleshooting procedures you will be able to reference them as needed. All important data should be backed up before starting this procedure. Remember that all hard disk data will be lost when you reformat. Check out this article on how to backup your data in Windows XP. Ensure that you save that backup to a CD, thumb drive, or external drive. Note whether you have the software CDs to extra devices such as printers or software you may have purchased. If you downloaded them, then ensure that you either can re-download them or backup those files so you are able to re-install them after you reformat your computer. You may also want to note the make and model of your devices. You can find these by searching for "device manager" from the start menu. Once inside the Device Manager, you can click on each category to see what devices are installed.
Consider defragging or using an anti-malware program first. Check to see if your PC manufacturer included a restore-disk. Write down or print out all instructions you need to reformat. Back up your data.