Depending on the OS you can Ctrl + Alt + Delete and it would change to a screen that you can enter a user name and password.
Enter the username "Administrator" and leave the password blank or try "admin" or "administrator"
If your able to login with those details you can go to
Control Panel > Admin Tools > Computer Management > Local Users and groups (On the left) > Users > and then right click the username you forgot the password for and click "Set Password"
Forgot Windows Password? Would you like to choose to reinstall the OS ? If not, then follow this tutorial to recover/remove the password.It is adjust to Windows XP/Vista/NT/2000/2003/2008/7.....
1.Download Windows Password Recovery utility from Password Recovery Official site :http://www.recoverlostpassword.com/products/windowspasswordrecoery.html
2.Decompress the Windows Password Recovery utility and note that there is an .ISO image file. Burn the image file onto an blank CD with the burner freely supported by Windows Password Recovery utility
3.Insert the newly created CD into the locked computer and re-boot it from the CD drive. 4.After launched the CD, a window pop up with all your account names(if you have several accounts) select one of the accounts that you have forgotten its password to reset it. Just one press, you have removed the password.