Are you using the windows providers or a multiplexing provider? If you are using multiplexing it could be that your first provider is the sql provider and an account was added to the sql provider with the same name as your windows account, but with a different password.
I have a testenvironment on a remote server. The database on the server is the same I use when I am developing on my local machine. I am using visual studios built in server when I run the site localy.
I am using windows authentication to login to admin and at first I had no problems to login, but at some point along the way it stopped working on localhost. The error message I get is "Login failed". The strange part is that I stil can login when I run the site remote.
I haven't changed anything in my config files so I am very frustrated right now...