Today I had a terrible day! I was working from my home office and all of a sudden my pc just shut down and rebooted all by itself. I wasn't even touching the tower of my HP Slimline desktop and it just shut down. I tried clearing my cookies and doing a disk cleanup but that didn't help. I would get it working for say ten minutes or so and it would do the same thing again. So, I decided to do a system restore hoping this might make a difference. No such luck, it kept happening over and over. Finally, it got to where it would not even fully get started back up before it would shut down again. Then I decided to run two different virus scans on it. When I did this it didn't reveal any viruses on either scan. It has been working ok for a couple hours now though. What could be going on? Has anyone else had this happen before?

This type of problems generally occur when there is a malware or a virus sitting in your boot memory. If you are using free version anti virus update them or use the trial version of famous anti virus to eliminate those malwares.

I've had very isolated instances of my PC rebooting on its own. I've had no serious operating system problems since using regularly updated Norton antivirus software, which is included with my Internet connection.

I have had a hard drive failure in the last couple of years, though - And that was no fun.

I dunno but I would seek professional help before you mess something up worse than it is already. I've done some serious damage in this kind of situation.

I assume you have an updated version of your antivirus software. With no infection and your computer is ok for a few hours, then your problem could be in the hardware. A piece of your hardware could malfunction once a certain operating temperature is attained.  It could be a capacitor or IC in the mobo...things like that.  A computer technician can help you.