A common occurance with all the family home computer users I support is that I will arrive on site, a problem is described that requires admin access but they have forgotten their admin password!
The way round this is to create another admin user within safe mode.
This works for Windows 7:
Restart the PC
Hold the F8 key down until you see the Windows Boot Menu
Choose Safe mode with Command Prompt
Wait for windows to boot and eventually you should be presented with a command prompt.
Add a new user (‘root’ in our case) using the following command:
net use root /add
Add the new user to the Administrators group:
net localgroup Administrators root /add
Delete the use from the limited users group:
net localgroup Users root /delete
Reboot the machine and you should now see a new user available at the logon screen. Logon with the new user and you can then change the password of the original admin user in the normal way (control panel, users).
You might also want to logoff, logon as original admin user and delete the new user you just created in the normal way (control panel, users) – just to tidy things up and not have multiple admin users hanging about.