Ive been having troubles signing in since getting a new PC but I think I found out why.
So if you cant sign in, try booting up you computer in Safe Mode with Networking (hold F8 at bootup) and see if you can sign in then. If you can, one of your services or startup programs in msconfig are blocking it. Turns out mine is Themes under the Service tab. Im trying to find a fix now.
See my thread about the problem:
http://forums.zune.net/458668/ShowPost.aspx