your PC is most likely overheating. most likely the cpu to be specific. open up your case, get your box running, and observe the fans and what they are doing. do a google search for Everest, it is a rather useful utility that will give you one hell of a lot of info about your box, including various temperature readings. your cpu (which I am guessing is a damn P4 Prescott) should be around 50C. you may need a new (or better) heatsink, and remember to use Arctic Silver thermal paste!