You’ll probably need to take it to a computer shop to have them remove the password. Depending on what kind of computer it is, and if it’s a password to get into Windows or just to get the machine to power on it might require actually opening the computer and moving jumpers around.
I would recommend being very careful with computers. If those computers are company computers and they contain customer information, medical data, etc. there could be state or federal laws which are being violated by you looking at the data on the machines. Personally I’d recommend just formatting the machine and installing a new copy of Windows before selling the PC. If the computer has a sticker on it with a license key the company that made the computer can give you a DVD which you can use to reinstall the OS for just a few dollars (you’ll only need one disk per company) or for free.