I know this sound outrageous and silly but trust me i have done it this way for years.

Take the CMOS battery out and put it some place safe, if you have a jumper that you can reset to clear you CMOS do so, then put your mother board in the freezer for atl east 24 hours.

You see, all your BIOS settings are stored in a CMOS chip and the only way to make sure you delete all the info stored in the chip now a days, is to hit them with a UV light, however they are a kind of volatile chip so using the CMOS reset jumper might not always work first time, you could be lucky of course and it work the first time or you could be unlucky and it doesnt work till the 100th time you reset it!

BUT,CMOS chips do not like the cold, or the heat for that matter, but its easer and safer to freeze it that it is to heat it up!

Give it a shot, also if you cant start your system up DONT flash the BIOS as it wont completley install untill you have started your system up COMPLETLEY, if you stop the prosess halfway through you will permantly damage you bios and hence your mo bo and will prob have to send it away to be re-flashed!!