i had similar problems. i would remote in with the correct username and password but it would dump me at the login screen as if my credentials were wrong. turns out they were..

a new machine was setup about a year ago for a user named sharon. it was purchased from Office Depot and i guess instead of deleting the "office depot" user and recreating the "sharon" user, i just renamed office depot to sharon. so if you were to search the user folders, you would find "office depot" instead of "sharon"... if you rename a user, the original name stays in the registry.

so instead of using sharon as the user name, but "office depot" instead, i was able to remote straight into the box. you might check to see if that helps you out any. it did for me.