comp resets when i try to play videos

[weewun]

for soem reason, my computer resets whenever i open a video file (tried .avi and .wmv) it even resets when i try to stream down videos from the internet. i can play music perfectly though. what could be causeing this? i posted a hijackthis log which came back clean, and i ran AVG antivirus which detected a couple things, fixed them but the problem persists... could there me any other reason? it used to work perfectly, then for no good reason i can think of it started doing this. cheers for any help!