When using EPiServer, especially when using it on a local dev PC, you may get an error such as:

"The trust relationship between the primary domain and the trusted domain failed"

EPiServer have a post about the error here, but they are only talking about a full server install environment. If you're using it locally, then it's more likely that you're just using a domain account to log on, when in fact you should be using one of the internal EPiServer user accounts. I confused myself by creating a 'soft' account called 'admin' and then wondering why I was getting the error... I was logging in as 'administrator' which is of course a full NT account :)