A friend of my who had been working on my pc for awhile has been stumped by its performance issues. I frequently have issues of the pc shutting down when playing video games or just accessing some videos on the net. Im trying to figure out if it just my video card failing or having issues because of a memory leak (he seems to think its a memory leak). Are their any free programs (or paid) that could determine what issues I am having? I did have recent help cleaning off a Trojan but my PC still has these issues.

Thanks!

You can use Process Explorer or a similar program to monitor memory useage.

... works on all windows systems too:
http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx