“Windows could not log you in. The trust relationship between this computer and the primary domain failed.”

If you have ever seen this error, and said to yourself:”*@*$&@@ !! How the !*$&@^@ did this happen?” then I am totally with you.

What this means:
Really, I have no idea. Nor why it happens, or why XP never had this issue but Vista and 7 do. Essentially, it’s a security thing when either the client or the server has reasons not to trust the other.

How to fix it:

1. Unplug the computer from the network, and log in using cached credentials.
2. Drop the computer from the domain by changing the domain name to a workgroup.
3. Reboot the computer.
4. Log in using the built-in administrator account (you should make sure you know this before dropping it from the domain! Otherwise, you will need to create this user by alternative means).
5. Join the computer back to the domain,
6. Reboot.

If this fails, you can try another method located here:
http://community.tigranetworks.co.uk/blogs/tim_long/archive/2009/05/16/windows-7-rc-failed-trust-relationship.aspx