I've seen a couple machines where when you try to log in and it says "Trust relationship between this workstation & primary domain failed." All you need to do is remove the computer from the domain and re-add it.
A couple different options:
1) disconnect the machine from the network and log in with a domain account (since you aren't talking to the server and you're using cached credentials, the trust relationship with the server is irrelevant)
2) log on as a local user
I spent a bunch of time trying to find a valid username and password of a local user today, but ended up going with option 1 after it occurred to me.