Cannot remove computer from old domain

I recently added a new computer to our SBS server. I then had to reinstall the SBS server, so the login from the old server no longer works. I am trying to use the "connect" application to add to the new user domain, but it is asking for an adminstrator password. My admin and password from the old domain obviously no longer works, and attempting to use a local login doesn't work either, I don't know if I have the login code wrong or what. Basically I'm asking how to remove this computer from the old domain, without having an Admin password. Is there a standard Win 7 admin password to give me the admin permission I need? I can't make a clean install, as Win 7 Pro was upgraded from factory Home Premium.