My HP Pavilion a300y recently started shutting down for "no reason" while I'm using it. The monitor goes dark and the computer does not respond to the mouse or keyboard. The small indicator light on the front of the PC stays yellow and I can hear the fan running, so it's not powered down completely. But the only way I can get it back is by powering off then restarting. When this problem first started happening it was only once in a while. Now it happens every few minutes, sometimes within a couple of minutes after starting the PC. I ran Norton Anti-Virus scan, MS Spyware remover and the Windows Disk Checker - no problems detected. The shutdown happens regardless of what software I'm using at the time (IE, Excel, Firefox - anything).
Any ideas on what's wrong? I suspect hard drive or power supply failure, but just a guess. Any suggestions will be appreciated.