If your concerned that only the hard drive is second hand, you can check the date on the hard drive itself. You will have to open the computer and pull out the hard drive.
I've checked a stack of hard drives I have here, and they all have manufacturing dates going back to 1999. Any new computer could reasonably expect to have a hard drive built in the last six months installed in it.
But there is no way to tell if its been used before without government grade forensic tools. The date stamped on the drive is the best way to check.
Jonathan
December 2006