New set of problems

Hi
Im hoping someone is able to help with this problem. I have had 3 computers go down in the last week - all at different locations. This one has me a bit confused.
For the last couple of weeks when I start the computer I get a strange "whirring/grind?" noise during the startup. Once it got to the desktop it all stopped and no problems then. Today I was in the middle of researching and the computer just turned itself off. When I tried to restart I got the BIOS beep and the page said that "all default settings had been saved to CMOS". It also mentioned the CPU but I missed the message (too quick). Then Windows started, got to the desktop, switched off again and then started beeping rapidly. The only way to turn it off is to hold the power button in for 10 seconds.
I managed to get it started again after about 2 hours and it shut down again after 5 minutes.
Im not sure what to look for - PSU (leaning towards), CPU or Mboard.

Specs
Intel P4 3GHz processor
160GB hard drive
80 GB hard drive
2GB DDR ram
256 mb AGP NVidea graphics card
Windows XP home

Thanks for any suggestions.
Sandra:cry

 

Whirling/ grinding is usually a bad hard drive and can manifest itself in many ways. Like the system not booting, etc. That would not cause the BIOS to be reset, that my be the onboard battery. Which could cause some issues with BIOS settings changed when the system rebooted.