Networking and Password

One of my client's laptop is no longer connected to our network due to
changes in DNS. Operating System is Vista Business. Although not connected
to our domain, the owner is the only user who can log on. As the
Administrator, I'd like to configure the laptop's DNS to enable it to rejoin
the domain. It won't let me as it won't recognize my administrator password.
User can't make any changes herself and we are unable to log on even to the
local computer. Every change that I'd like to do asks for administrator's
username and password which won't be authenticated. I tried to re-install
the operating system but again it wouldn't allow me to without an
administrator's username and password. I'm stuck and any help out there will
be greatly appreciated.