c1atsite - I've told you what will fix your computer. You just have to reinstall and use nLite or switch to linux to get that older computer of your's to be more responsive.
That goes for all computers that have been running windows for a few years without a reformat and clean install. There's no way around this. All OSes, if you regularly use them, will start to have a bit of corruption and errors in the system files (and regristry for windows), windows just happens to have more problems with this than other OSes do. I could explain why that is, but I'd lose everyone.
In short, if you want to fix it reinstall windows. If the computer is a few years old use nLite to assist in this and lighten up whats installed. Windows has a lot of unnecessary things that eat up resources - getting rid of all forms of explorer and replacing them with more functional programs that looks nicer and use less ram and cpu is a good example. (There are some truly beautiful desktop shells for windows, too)
You also need to do disk and memory checks just to make sure nothing is failing. If all is well, look up your motherboard, find out the type of ram it uses and the max the motherboard supports. With an older PC you should be able to get some extra ram for it for fairly cheap and that will help the slow down when using it (and opeing programs) to some extent.
