Is there a way to completely erase my computer and start over?

I am worried I have viruses on my computer and I have anti spyware and firewall and all of that but I know that some viruses can hide, re install themselves, or simply not be deleted upon detection. Is there a way for me to completely erase everything thing I have done on my computer since the day I purchased it in order to start with a clean slate. I dont have anything saved of any importance except for a couple things which i can save on a flash drive and re load later. Is there a way to do this? (I have windows XP is that means anything)