If you can't get in with one of the backdoor passwords, it's time to open up your computer.

Remove a jumper
There's a jumper on your motherboard that you'll need to identify and remove. Most motherboards make your job easier by actually labeling the correct jumper as "BIOS config" or something similar.

Remove the jumper, and then turn the computer on. In some cases, you will have to remove the jumper while the system is powered up, though to minimize risk, try doing this first with the computer off.

Remove the internal battery
If you are not able to locate the jumper or believe it doesn't exist, your next option is to remove the internal battery (usually looks like some variation of watch battery) from the motherboard, and unplug the computer.

Without power to retain the CMOS information, most computers will clear it anywhere between 10 seconds to 24 hours. Be sure and let the computer rest without any power whatsoever for the duration of this period before ruling out that this method will not work.

Overload the keyboard buffer
Overloading the keyboard buffer can crash the password routine and let the computer boot. Try this by pressing the ESC key repeatedly, possibly more than 100 times.

Remove the hard drive
If you've got crucial data on your system, need it urgently, and can't get past the BIOS. Remember that you can always remove the hard drive with the data and access it from another system. The BIOS password only pertains to the motherboard.