There are many ways to do this, I mean to unlock a locked computer without knowing its password. Well, Here I list out few of them.
First Trick :
Open your computer’s case. Search for a small battery on the mother board, Which will be of a size not more than a coin, If you remove the battery for about 10 seconds, It resets the BIOS settings along with the Boot or BIOS Password. When you restart your computer, It will ask whether if you want to load default Cmos settings or not. Say YES. By doing this , you can reset the password of your computer , It will also ask you for the Time and Date, As it will also be reset along with the Password.
Second Trick :
Is the above trick working for you? Sometimes It happens, Alright here is an alternate solution,
Start up the computer and as soon as it start booting up click F8 repeatedly. A menu will come up asking you to start PC in Safe mode, You “start it in safe mode.” The computer will start normally except for a few small differences and you should be able to log in without the password. Once your in, go to the control panel and remove the password.(This do not work for all Operating systems)
Third Trick :
If both of the above Tricks came useless in solving your Problem then you can go for the most Efficient way for this, You just need to Reinstall your Windows Copy on your system with a CD. Sometimes repairing Windows may also reset your password. (Not sure about it)