If you truly want to completely erase your OS and start over, there is software called Wipe Drive. It costs about 50.00US. I have used this a couple of times and it completely erases everything. Ensure that you have your OS disc to reinstall.
After you clean your hard drive, re-install OS, there should be some kind of disc that came with your computer for your drivers and utilities. Re-install your internet software. Download and install all updates. Then re-install all your other programs.
It's not as bad as it sounds. However if you do choose this option be sure, it erases EVERYTHING!! Best of luck!!!