um....
first of all, you can't 'downgrade' an OS ontop of the other one.
The only thing you can do, is create a new partition (this can be done w/ the windows installation disk) to put the OS on, or simply reformat the existing hard drive (assuming you have all your data backed up).
when the computer turns on, the BIOS checks all the hardware to see whats there. this is called the POST. Does this list show your hard drive?
Make sure its plugged all the way in, and has the necessary power connections hooked up