There are lots of little things that slow down a computer, mostly it is programs that are running, but day to day clutter can slow your computer down too.
A good thing to try is to download a good program like Ccleaner or Reginout. I personally Use Reginout because i don't believe on free products.
It will analyze your computer, and clean up all that it can safely clean, inducing your registry.
It works well, and is 100% reliable.