My computer "chugs down" and locks up for brief periods - someone said it was an easy fix - HELP!?

Ok - so I have an older Compaq Pentium 4 - it worked fine untill a roomate got ahold of it. I used to be able to run games like Elder scrolls 3 and the Sims 2, but now, even running a video is difficult. It will work fine for a while (like 10 - 20 min) then it will lock up for a min. Then it's fine again for a few. Looking at the CPU usage shows spikes to 100%. Someone told me that everything is being funneled through a single (something I don't remember) and it was an easy fix. I've run various virus scans to no avail.