Besides viruses, modern computers accumulate so many junk files that just aren't used anymore or needed. The longer you run your computer, the more these build up. In order to combat this crud, try using a program like PC Health Boost. I downloaded it onto my grandmother's computer and now she wants me to use it to fix her other computers!
You can find it at www.pchealthboost.com
Have you checked to see if it's infected with viruses? That usually makes them run slow.