I can vouch for Malke's suggestion. I've seen it used many times successfully. The best option is to reset the password, however. It is a rather "geeky" tool; not a nice Windows graphical user interface. You might want to check this page for a demonstration of its use, with screenshots provided in the article:
Reset lost Windows passwords with Offline Registry Editor | Microsoft Windows | TechRepublic.com
There is also a video about using this tool here:
Video: Reset Windows passwords with the Offline NT Password and Registry Editor | IT Dojo | TechRepublic.com