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Skindred

I'm going to take a SWAG and say your primary BIOS is corrupted and the system is reading a backup BIOS off the HDD. The very same thing happened to me and this was the cause. It only occurred once, but further investigation showed BIOS was using an earlier version than what I had flashed BIOS to the last time I updated it.

To fix things, I just reflashed BIOS to the latest version. Let me say that in my case the system ran fine with the older BIOS.

How old is your CMOS battery? It could be dead or dying, too.

its pretty new.. i just bought the pc last April.. I have a Gigabyte motherboard (cant give you the model though coz i'm at work). Altough, after i bought it, i've already had the PSU replaced thrice. Could a bad PSU be a cause for this?

Hi Red, sorry for the delay,

It's not impossible for a faulty PSU to cause motherboard problems, but have you tried resetting your CMOS?

Really depends on the model of the board, but popping the battery out for a little while usually does the trick. Some boards will have a small jumper very near to the battery. The CMOS jumper has 3 pins with the jumper covering two of them. Slide the jumper off, and place it on the opposite two pins and start the PC. Put the jumper back and your PC should then start normally.

With most of the newer Gigabyte motherboards though, there is usually a small button very near to the battery, often blue labelled something like "CMOS SW".

EDIT: Marked as Answered.