Reading Master Boot Record

Hello all,
After a drunken discussion with a friend I've foolishly agreed to accept a challenge that I've really no idea how to solve. I need to write a program which will prevent a PC booting up depending on the status of a bit in the master boot record. For example if it's set to '1' the PC will boot normally otherwise the PC should not boot but instead display an error message. I'm a decent programmer but I'm not used to such low level stuff so don't have a clue on how to approach it. Personally I don't think it's possible. Could people please tell me what their thoughts are and how I could get started? Thanks in advance.

I think its possible if you have the ability to rewrite your operating systems boot sector. Not something id like to mess with personally.
What os does this have to work on?

There is such a way by using assembly. U can just make a function out of it in assembly. Close to begining of ur code write "extern writembr();" or what function name u chose to have. Make an object out of it, link the object to ur exe. The assembly code itself I may post later becouse I have to write the code.... I sugest using NASM (netwide - assembler) for the assembly section and link it (method depending on what compiler u are using).