Dieconnect the hard drive.
Disconnect all USB devices from the PC This includes mouse, keyboard, monitor USB Hub if you have them.
Try to boot up, let it go as far as it will. Power down.
Reconnect mouse, keyboard. Try to boot again. If this works, your USB ports may have "locked up".
Re-Attach HD. If it fails now, the problem is your HD.
If this doesn't work, open the case and try to find any cable, memory, expansion card, etc that you might have disturbed in your cleaning.