BleepingComputer.com: Custom Built PC, randomly restarts

I have a custom built PC, (built in 2006), that recently has had a slew of issues. In the past year, I have barely used the computer, and I just recently decided to start using it again.

When I started it up, I would get a "CMOS Checksum Bad Error", assuming the CMOS battery was the culprit, I replaced it.

After I did this, I started it up and found that it would randomly restart or freeze. And then sometimes when it would restart I would get error saying there was a bad disk, and to install one and press enter. This would only pop up occasionally.

So now, i have taken out all the components, cleaned everything, and put it all back together. And I still get the random restart issue.

I have Rosewill 550W PSU and a ASUS A8n5x motherboard. I'm thinking it may be a PSU issue or possible a dying motherboard, any thoughts?

mine did that for some reason a little bit back and all i had to do was go into the bios and hit the "set all back to defaults" button and it worked....just a simple possible thing to try