In the last few days (not particularly hot weather) my PC has turned off (instant power off) about 4 times for seemingly no reason (i.e. not doing one particular task, once was playing COD4). When I restart it the CPU fan is running very fast and once I get back to windows (if it lets me, normally i need to leave it off for 10 mins) I check the CPU temp and its at about 67 or a little above - guess it would have been higher at the shut off point). Leaving the side panel off has helped stop it happening. I have an Athlon X2 4400 with stock cooler. System has been running for good few years. does it sound like the cpu turning off the system? Can it do that? Would cleaning the heatsink be a good solution?
My mobo is running at 58 degrees which ASUS PC Probe II is telling me is high (alerting me by lighting it up in red) but it isnt going any higher than that.
Try removing the stock cooler, cleaning it, removing the thermal paste (with Artic Cleaner or something like that) and then using some Artic Silver 5 o OCZ Freeze. I would get some more fans into your case (or some with higher CFM).
Well it could be some of your fans are dead/dieing, so I would check all of them - especially the ones on the graphics card and CPU. Also, if it's really dusty in there that can have an impact on heat as well.
I don't think I've ever heard of a heatsink-cpu 'connection' going bad over time, so I have my doubts about that. Have you tried to overclock recently?
Again though, fire it up and see what the fans are doing.
the asus probe is user configured - red means nothing and 58c is ok
you will get instablity above 65-70c sometimes
clean the computer!
67 is high but shut off is higher -- crash could be from many things
Overheating. CPU's cool FAST (I had a Pentium D drop 20 degrees in 2 minutes once; did a test where i turned off all my fans, made the temp go to 80, and turn them back on) when not doing anything. If you get 67 when you get back to windows, you were likely in the mid 70's when you shut down.