I will take a guess that it is the BIOS password you have forgotten, if so it is relatively easy to reset but it does involve opening the case. On most mother boards there is a jumper that needs to be closed (refer to the motherboard's manual) or look very closley at all the jumpers on the board itself to see if there is one marked "BIOS reset" or similar, simply close or open (depending on motherboard) then put it back the way you found it, connect everything back up again and boot. (remember to earth yourself before touching anything inside your PC by touching the case first!) Hope this helps!