I'm no computer expert, but I can tell you what "Windows Vista for Dummies" (publisher Wiley Publishing Inc.) says about the problem. These hints refer to Vista.
Retrieving a Forgotten Password
Let another user reset your password. Anybody with an Administrator account on your PC can reset your password. Have that person choose Control Panel from the Start menu, choose User Accounts and Family Safety, and click User Accounts. There, they can choose Manage Another Account, click your account name, and choose Remove Password, letting you log in.
If this doesn't work, then you're in sad shape, unfortunately. You might have to pay a password recovery specialist. You can find one by entering "recover password" on Google or another search engine.
That's what the book says. I'll keep my fingers crossed for you.