Any good diagnostic software for "sick" PCs?

My gaming buddy has been having some trouble with his machine lagging really bad and even freezing up at times during out sessions. What is the best (preferably freeware) diagnostic software tool out there, to see if the video card, processor and/or something else is not working as it should?

SiSoft Sandra is a good bet. There is a free Lite version that should do all the basic diagnostics and benchmarking. It won’t do much in the way of diagnosing OS problems though, and I’m not aware of any good Windows diagnostic programs that are free.