Look for your mobo manual. There should be a cmos jumper and if you take it out and wait 30 seconds, then put it back on then bios will reset. It is like a little square piece that goes over two of the three total prongs. Usually at the bottom corner of your motherboard. It should be in the manual.
It works on my Sisters HP Pavillion a810n which is basically the same as the a420n but, it has a different motherboard and a little but faster CPU. So I think it is my motherboard.
In the a810n the maker of the motherboard is SiS and in the a420n it's VIA.