[ematulac]

Sorry, the above post applies to your second computer.

For the other computer, open the case and make sure that all the power and ribbon cables going to your CD and Hard drives are seated properly. They can loosen up over time. If you have spare ribbon cables handy, replace them. They tend to become frayed or damaged with constant handling.

If you recently added hardware to the computer, then remove it and see if that solves the problem. You may have installed more hardware than your power supply can handle.

If it's neither of those, then your IDE controller card may be dying. You can buy an inexpensive replacement at your local computer parts store. If it's built in to your motherboard, disable it in your BIOS settings when you install the new one.