Best thing to do is get some benchmark/diagnostic software to test the graphics card, CPU and memory. There's a couple I can think of, which are. Burn In Test and PC Mark. Links below for each. Just stress the hell out of the component's and see what causes the system to fall over.

BurnIn Test : http://www.passmark.com/ftp/bitpro.exe
PC Mark : http://www.futuremark.com/download/?pcmark04.shtml