I always think if your pc is full of crap and its still workng well then dont re-format cause you might inherit a problem or two.
instead you could just download a program to clear all the rubbish out and get the free trial od Disk Keeper to properly de-fragment your drive.
go to the search part through the start menu and type for a specific file type to see what you have , like
*.mp3.........the asteris means everything.mp3 so it will search the drive for all of the mp3 files.
i usually search for these
*.tmp
*.log
*.bk
*.avi
i look through them and delete what i dont want.
Re-installation of windows is o substiture for good housekeeping :P