You cannot logon interactively on a Windows computer and you don’t have domain account with the right privileges, here are some techniques
to reset a lost/forget admin or local account’s password:
1- Paying method: If you are covered by the MS software assurance, you are eligible to the MDOP “Microsoft Desktop Optimization Pack”: http://www.microsoft.com/en-us/windows/enterprise/products-and-technologies/mdop/default.aspx
In that case, you can use the MS DaRT to burn a boot CD in order to recover your lost password. The Emergency Repair CD is based on well-known Winternals ERD toolset. The ERD contain the “locksmith” utility in order to recover your local accounts.
2- Free method: The other solution that requires no extra tool (but need the Windows DVD) is the method based on Utilman.exe substitution: http://sburke.eu/blog/2012/01/reset-windows-passwords-for-vista-7-and-2008-server/
3- Free method: Another method based on USB or Boot CD (based on linux tools): http://pogostick.net/~pnh/ntpasswd/