Were you ever able to access your bios? Whoever built the system might have password protected it, but it should still ask for a password not just give you a black screen, unless the builder did something else to lock you out. Who built it?
It might be better to have some more information before you try the following.
Assuming the problem is a bios setting as opposed to a dead MB you can reset your cmos RAM to "bios defaults" and piece things back together from there.
With the computer off and power supply unplugged:
There will be a bios reset 3 pin header on your motherboard. (check your instruction book or get the MB diagram from your manufactureres website.) Pull the shorting strap off and put it on the the other side of the header. wait 30 seconds then put it back where it was.
WARNING DO NOT POWER UP WITH THE JUMPER IN THE CLEAR POSITION IT CAN WIPE YOUR BIOS!
Plug it back in and start the computer. Enter bios setup immediatley. With all the settings defaulted going to a windows boot could hose your install. Put anything like on board sound, raid, etc back where it was. Your system should be ok now and you can go back to trouble shooting the original problem.
Of course the catch here is if you can't get into your bios you have no idea how it's configured.
I think I saw a tool out there somewhere that will let you read your bios settings from windows. Anyone rememebr the name?