At first I thought he was talking about the BIOS password, which is easy to reset by simply replacing a password jumper on the motherboard.
But then I realized he's too stupid to even know what BIOS is.
Isn't there a format option or a way to reach FDISK from BIOS?
You could go into BIOS and set the boot priority to your CD/DVD drive and put in the Windows OS disk, restart the computer, and just reformat everything.
Considering it's from somebody else, any personal data remaining on the HDD shouldn't matter since doing this will erase everything.