I can not open computer, because someone reset

1st Method:
Start system and when you see Windows Welcome screen / Login screen, press ctrl+alt+del keys twice and it'll show Classic Login box. Now type Administrator (without quotes) in Username and leave Password field blank. Now press Enter and you should be able to log in Windows.
Now you can reset your account password from Control Panel - User Accounts.
Same thing can be done using Safe Mode. In Safe Mode Windows will show this in-built Administrator account in Login screen.
2th Method:
Windows password Recovery software:
for example:Windows Password Recovery Tool 3.0 . It is easy and convenient to use.
http://www.windowspasswordsrecovery.com

There’s a way to reset the password and it doesn’t involve reformatting and reinstalling Windows. The solution is called Windows Password Reset 6.0. It can reset almost all Windows passwords in seconds. If you want to know how to use it, please visit it http://www.resetwindowspassword.com

Remove the battery from the motherboard, that will reset the BIOS password. If you're talking about a Windows password to login, then you need this: http://www.howtogeek.com/howto/windows-vista/change-your-forgotten-windows-password-with-the-linux-system-rescue-cd/