That's also a normal temperature for the CPU. :)
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Have you registered your version of Vista, due to the fact that it will restart after two hours of use if not registered/activated?
Right click Computer > properties and see if it's activated, if not that's why, as it won't warn you about the restart.
That happened to me when I had to run Vista. I just did a fresh Vista installation and it fixed it. Never found out what caused it.
Nothing wrong with your CPU temp therefore you don't need to look into buying a new fan. As matt said isn't there a little yellow box? That asks you to restart now or postpone the restart.
Maybe you have postponed the restart, gone away from the computer, the time in which you asked it to postpone has expired therefore restarted.
After you have noticed it restart do you get any notifications upon load-up saying installing updates, step 1 - 3 etc?? - If not it might not be a update problem hmhm ;/
another possible issue is driver issues, I had this once on XP x64, two drivers weren't compatible with eachother, causing a restart, think back to when it started happening and think about new programs, parts, etc which you have added/installed since it's happened, might be an issue to do with that.
As I said, it usually has the yellow box, but today it never done that and it restarted. ;)
Yea, Dave I haven't upgraded or installed anything new in the past 48 hours.
Also, earlier my anti virus caught an IRCBOT, could this crash my PC?
IRCBOT won't cause your system to crash. I think i have pulled something up im just reading it at the mo
IRCBOT info - http://en.wikipedia.org/wiki/Internet_Relay_Chat_bot