If you royally screw up your BIOS settings, you can almost always reset the CMOS by removing the CMOS battery (about a nickel in size, pretty easy to find). Some newer motherboards have jumpers or even buttons for doing this

EDIT: Just for clarification - MAKE SURE THE COMPUTER IS OFF

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1982 240D, sold 9/17/2008
1987 300D TurboW124.133 - 603.960, 722.317 - Smoke Silver Metallic / Medium Red (702/177), acquired 8/15/2009
262,715 and counting