Recently at work I installed a little 9GB drive that the IT guy pulled from another computer in my department, and since then, every other power-up results in no response from the mouse or keyboard. (Well, the peripherals act normal, but the PC doesn't see it.) It's weird what power a HD has over the PC.
You problem- my first guess would a common error when taking an HD from one PC to another: could be that you have both HDs set as master on the same IDE channel (or, depending on your setup perhaps you have two slave drives on one channel). Check the HD jumpers. (why it worked for a while I don't know-- did you or Mo install something on the transplanted HD that runs at startup?)
If that's not the problem, bang your head against CMOS for a little while to see if the PC has correct notions about which HDs are where. Also, can you boot from floppy? (If you're like me, you don't have an bootable floppy sitting around, but you can borrow or make one at work, maybe.) Good luck!
Try VGA mode... I KNOW it sounds stupid, but had similar problem and it was the video card in the end.
Try bootdisk.com You can make boot disks for most OS's and lots of trouble shooting information