Lost admin account after windows update

My development machine was rebooted after a windows update recently and now I can't access my admin account.

I never reboot my machine except when forced by WU so I actually don't know if the problem is related to WU. Don't remember when I last used the admin account but it is probably a week or 2 ago.

Normally I run in a normal user account and keep an admin command prompt open to deal with admin tasks.

What happens now is:

• If I do something that triggers the UAC and enter user ".\admin" and password I get the message
  Logon failure: unknown user or bad password




• If I do RUNAS /user:.\admin cmd I get the message
  RUNAS ERROR: Unable to acquire user password

Any idea what the problem is?

It is not a big problem since I can call in a domain admin but I am curious about what has happened and why UAC and RUNAS show different error messages.

Answer

The SAM file might have been damaged.
Explanation on [SAM]: Where are Windows 7 Passwords Stored? is available here.