Have you cleaned the computer? Inside the case and in the OS?
To clean in the case get some canned air and a few q-tips. Unplug everything and take the case outside. Open the case, keeping your hand on the bare metal to discharge any static. Use a q-tip to hold the cooling fans and blow the case out.
To clean the OS I use CCleaner, EasyClean, CleanUp4.5.
Use EasyCleaner only if you are comfortable working inside the OS itself, IE Windows.
You can set CleanUp4.5 in options to Thorough or check all the boxes and do a custom clean. It will tell you what you are removing and how much space you are getting back. On one computer I reclaimed 82GB of space that was full of garbage, dead files and registry keys.
CCleaner is very good for cleaning as well, it cleans the crap files and you can also clean the registry using the same program.
Get memtest and check out your RAM, you could have a bad RAM stick.