Video Converting Help, Please!

About 2 months ago, I was converting videos just fine. I was using a program
that converts any video to iPod mp4. I used to leave it overnight converting sometimes. One day, I left it overnight, and when I went to convert a video,
the computer just restarted by itself. It has been going on 2 months now, and the only things that I can convert are youtube videos and small spongebob episoded but no movies. I was converting with about 2GB of memory, now I have about 6GB and it is still restarting. I have tried system restore and it has failed. I have uninstalled the program and downloaded others and it still does the same thing. And it's not a virus because I checked.
What can it be?