You do realize that you can reinstall the OS and NOT overwrite the existing OS/files, right? You basically install a 2nd OS, even if it's the same version as the current one. Then when you install, two boot options will appear and you choose the 2nd one. Now, all the software that you want would have to be reinstalled as well but in most cases, that's a good thing because you will be working with a fresh install, not the 5000 programs that were installed with 3500 of them uninstalled... (a bit overstated, but you get the point).
Obviously if you do this, be very careful on what options you choose when you install - you do NOT want to have it reformat your hard drive or overwrite the previous OS install (overwriting probably wouldn't help as you'd still have all the failed installs that are currently there).
Just a suggestion. I've done that on a pretty regular basis up until I got Win 7 - haven't had the need as much but I am pretty good with maintaining computers. (I've been doing it longer than many of our userbase has been alive).
Good luck!
OS