take the HDD, put into another PC, and DL a professional HDD cleaner. Use and high level multilevel 37 mixed pass of erasing. That should be decent.

What it does is rename all files and extensions on your hdd, erase your hdd, and write garbaged over and over and over 37x times using different combinations and format into different sectors. Depends on HDD size and the PC's speed. Should take up to couple days. I know my 500GB SATA took 5 days to do that on my old 3.2ghz P4.

Then put that HDD back to the PC and reinstall the OEM OS and applications before selling/returning it.