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.
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.