Some virus programs are also known for slowing the crap out of computers. Norton and mcafee are the worst. In recent years they have gotten better though as I have heard. I use AVG. Another thing is how many start up programs are running on your computer (the ones that launch when you turn on the pc) some people have soooo many and then wonder why thier computer is running slow these programs are constantly running in the background?

and like everyone else said run a defrag at least once a week. If you have never done one or have not done one in awhile it will take forever so do it while you are sleeping. Empty your recycle bin, cache, temp internet files, and uninstall any programs you are not using. Keep in mind when you bought the computer it probably came with a bunch of junk you just do not need (get it off).

if it is a dell computer (sorry) I hate dell computers....

and don't forget to keep everything on your PC up to date as well.