Re-check all the connections going to the hard drive. Connect only the hard drive and disconnect the DVD-ROM drive that you connected. Try to boot the system with only the one HD and see what happens.
You may have overlooked the jumpers. Assuming you have IDE drives, I would make sure you have the jumpers in back of the hard drive and optical drive set correctly. Set one to master and the other to slave or both to cable select.
I've had 7 PCs in my time, two of them had hard drives which failed. Both of those were Maxtor - and haven't touched a Maxtor HD since then.
Don't know if they've improved their reliability any since then, but I still wouldn't buy one on principle for all the hassle they've caused me in the past