If it's an Acer 5100, it's the HDD power connection. Let us know if it's the 5100...
If you have a bootdisk or rescue disk, try running chkdsk. If you can't get to that, or it hangs, the guy probably screwed you with a bad/dead HDD.... Was there no way to check if it was working correctly before purchasing? If not, I wouldn't have purchased it.
If you paid by check, have the bank stop the check, likewise if with a credit card...