BSOD when running cpu intensive software

Hi all Hope someone can help. I've had a BSOD for several months, but very infrequently (once every couple of weeks). Stupidly I've just been ignoring it, but I'm trying to render a HD video in Sony Vegas and I now keep getting the BSOD at every attempt. I also get the same problem when trying to play Mass Effect 2, so I'm assuming it happens when the system is being utilised to its fullest....