What to do if i accidentally reset the computer account

I don't believe this is a bug.

The difference is that the account lock out policy applies to new sessions with new auth and login under default circumstances.

In the case of a locked workstation, there's already an active, logged-in session on the box.

This can apparently be changed by the "ForceUnlockLogon" registry setting. Toggling this requires a workstation unlocking to get re-auth from the DC and thereby avoids the use of old, cached credentials.

In the case of a locked LOCAL account in the workstation, you can add the following registry value into HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\policies:

DisablePasswordCaching DWORD 00000001

Quick googling turned up this paper by MS over at sysoptools: http://www.sysoptools.com/support/files/Account%20Passwords%20and%20Policies%20In-Depth.doc

It has details relating to logons, account lockout policy, and discussions of settings that can come into play.

Additional Note re: #3 (also covered in the linked paper): a locked-out user may still gain access if their still-valid kerberos ticket has not expired.

You dont have do disconnect the client from the domain first. Log on with your domain admin-user, and bring up computer management (Right-click "My computer"-->"Manage")

In Computer Management, go to "Local Users and Groups"-->Users. Right-click the administrator and choose "Set Password". Also, make sure the account is enabled if you want to use it (In Vista, by default, the account is disabled).