There are a few programs which will allow you to change a password as long as you know the account name. There are others which will enumerate accounts if you do not know the account name.
I hate to sound the way I know I’m going to, but why do you need to “break the log in password”? If this is a corporate setting, and you cannot (for whatever reason) contact the Help Desk/Service Desk regarding a lost or forgotten password, please be aware that there may be serious repercussions concerning unauthorized system access.
That having been said, I would suggest the Ultimate Boot CD for Windows (http:\\www.ubcd4win.com) as a good first source. Also look at L0pthcrack (if you can still find it) or the newer ophtcrack (from Sourceforge here: http://sourceforge.net/projects/ophcrack/).