If the system doesn't post, check your power supply connections to the drive. I use a boot disk or cd from the hardrive manufacturer. Maxblast 5.0 is a free download; other drives have similar software. You will need to format the drive anyway and reinstall windows or linux. The old compaq windows may work if the owner made any backup dvd's. Ubuntu is free if you just want to surf the net. You can also reset the bios which may help. With the power supply unplugged, either remove the motherboard battery for one minute, or move the cmos jumper over one position, keeping two out of the three pins covered, then return to the original position.