What are some of the best programs for fixing problems with your PC?

I'm not concerned about viruses and spyware. I have a good program for taking care of those. But it seems like I have a lot of quirky problems, like programs hanging up, opening slowly, etc. I already ran defrag. I just need a good program for identifying and fixing problems with my OS and software. I'd prefer freeware, but will consider pay software, as well.