How can i totally erase everything from my computer and start fresh like the day i bought it?

I have like 50 gigs of crap ive downloaded and junk and its acting up running slow, probably has viruses and other junk on it. some of the registers are bad media player wont work anymore its a freakin mess i just wanna wipe it and start fresh. Is there a way to do this without having to go out and buy another windows disk. i lost whatever disk i created when i bought it so im probably SOL there but i have backed up all of my important documents and pictures to a solid state memory storage device. any suggestions?