If this is a computer that came with Vista preinstalled, the maker of the computer should have included options to recover the installation? Contact the computer maker for the instructions on reinstalling Vista.
If you want to preserve the data on the system, there are several ways to recover if you did not create a password reset disk when you set the password on that admin account.
This procedure is free, but can be complex and requires advanced computer skills.
Offline NT Password & Registry Editor:
http://home.eunet.no/~pnordahl/ntpasswd/main.html
There is also software available that you can purchase that will solve the problem, easily.
The program that I recommend is the first one, it is very easy to create a recovery disk that you can use on any Windows PC. These recovery programs usually cost about 30-40 USD.
@PC: Windows Password Recovery Software. Reset Administrator Password. XP VISTA 2003 2000 NT.:
http://www.password-changer.com/
Windows Password Recovery Key - Resets the Administrator account password:
http://tinyurl.com/2qr7oc
Hope this helps.
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Ronnie Vernon
Microsoft MVP
Windows Desktop Experience