Since you have backed up all your important data, and if you have the programs you need to install, and you have your os disc, then the quickest way to remedy that slowness is to repartition the drive, reformat it, then reinstall your os on it. Then all you'd have to do then is just reinstall your programs, and then place your data back into those folders.
Then again, you could always uninstall one program at a time, defrag the computer, and also do a registry cleaning. Chances are, your registry is full of bad entries and it's slowing down your computer. Try this FREE registry cleaner:Eusing Registry Cleanerhttp://www.eusing.com/free_registry_cleane...try_cleaner.htm
Eusing registry cleaner is a fast and FREE, and most importantly, very good at cleaning out bad registry entries. Even if you use other registry cleaners, you should definitely add this one to your list.
It has a function to allow you to backup your registry 1st, and the function to restore it too, just in case. But I never had any problems cleaning any of my registries.
But if you don't want to reinstall an os, I'd start uninstalling program after program, reboot, then clean the registry, then reboot,,, then uninstall more programs, reboot, then registry clean again... until the computer works as normal.