Assuming you're using Windows XP/Vista/7
2)After POST screen press F8 before the Windows splash screen.
3)Chose SAFE MODE.
4)Go to computer management (right click on computer-> click manage) & reset administrator account password.
This requires you to have another account on the same computer.
If it does not work, you may try one of these methods listed here http/4sysops.com/archives/three-ways-to-reset-a-windows-vista-admin-password/
We don't give password advice here in case we help someone hack someone else's computer. Your best route is to take the machine to a professional computer fixer, along with some positive proof of ownership, and pay to have the system opened.
An admin password only works on the system's Administrator account - not the User Accounts. It is extremely rare for anyone to forget a password they need every time they log in.