Accessing a Windows computer without knowing the password is fairly simple with this free tool called Kon-Boot .There are alternatives like Ophcrack etc, but those rely on grabbing the SAM hashes and cracking those. What sets Kon-Boot apart is that is modifies the kernel on-the-fly while booting (everything is done virtually – without any interferences with physical system changes) and allows you to log into any account without entering a password. All you have to do is insert a boot (cd or floppy) disk burned with Kon-boot software(110kb) in to the computer and boot up.
Kon-boot which was initially started as a small project for Linux (mainly Ubuntu),where it allows to log into a Linux system as ‘root’ user without typing the correct password or to elevate privileges from current user to root. Now it was moved to windows platform where it enables Windows users to login to any password protected machine profile without any knowledge of the password.
This program works with the following versions of Windows: XP (SP1, SP2, SP3), Vista (Business, Ultimate), 2000, Server 2003 and 2008, and Windows 7. Kon-Boot also allows you to boot Linux (distributions: Ubuntu, Gentoo, Debian and Fedora) without a password as well.