This tip came from an actual problem we had to solve recently where a Windows 7 computer turned up but that you simply couldn't logon to, given the very simple password that the user had always used.
We tried everything we could and couldn't log in. We don't know what went wrong. But the solution, although it took a few steps wasn't terribly difficult. It wasn't one that I'd recommend an end user try because it involves fiddling around with the command line and tools that you wouldn't normally use. But we did end up solving the problem.
We've included a link on our website to the article we found that helped us solve the problem and it worked one hundred percent. We were able to reset the computer's password back to what the user thought it should be with no ill effects of any kind. It's nice that we were able to help this user, but it does point out an interesting security problem, and that is on Windows and any personal computer, if a thief is able to touch your computer it's possible for them given enough time and effort to break into your computer and get your data.
This is a good reason to ensure that you have good passwords on all your accounts, and that you have a good backup of your computer at all times. Remember if someone can touch your computer most likely with enough effort they can break into it.