|I have used this tool for years. It's called Offline NT Password & Registry Editor, and works for Windows NT/2k/XP/Vista. You can access it here:|
Just follow the instructions and note that it's usually safer and more reliable to delete the admin password rather than to change it. Afterwards, you should of course set a new admin password using the Windows "User Accounts" applet in the control panel.
Also note that if you have any files you've asked Windows to encrypt, you'll no longer have access to them without the original password.