Hi
I would agree with Lsdmeasap regarding the setup of boot drive on the first SATA_0 port. It is always advisable and usually the fastest available too.
I would do an extended clear of the CMOS in case it is old data that is causing the problem.
Remove the power cable from the mains supply and then press the power switch on the case for a few seconds just to drain any residual energy in the PSU capacitors.
Once done remove the motherboard battery for at least one hour before replacing it.
Next plug back into the mains supply and boot.
You will now need to enter the BIOS by pressing DEL and load Optimised BIOS Defaults.
Make any other changes to the BIOS settings to suit your self like disabling the floppy drive, disabling the full screen logo and making the HDD the primary boot device and then press F10 to save and exit.
If you have any doubts as to wherther the OS is corrupt it would mbe best to wipe it clean and do a fresh install. It would also help to make it faster and get rid of any old files etc that are hanging around.