I have a PC running with the same chipset (P4P800 865PE). Even though it was running fine, I replaced the 6 year-old power supply last year just to keep it going. It might just mean you need a new power supply.
Bring it to a PC shop and they can usually test it for you. Buy a new one if it's bad.
Chances are that PC has given you at least 6 good years?
The other option... You now have a good excuse to upgrade?