So, my kid's computer has Windows XP, with no disks. I'm pretty sure it has some viruses on the computer, of the type that don't seem to come out despite trying multiple anti-viral programs on it. I'm ready to just format it and start over again. I have a license sticker on the box, but no media to install it. I strongly suspect the license is OEM, but I don't have any proof. What suggestions would you have?

I should say, the computer originally belonged to a business, is probably 6 years old, and I am willing to pay a small charge if required. I don't want to change the OS installed either.