Account used elsewhere when its not?

I recently decided to start up Gmod again after taking a rather long break from it. I tried to join a multiplayer server and get a pop up message saying “This Steam account has been used to log in from another computer. To continue using Steam, you need to log in again.” I double checked and I am not logged in on any other computer in the house. I can play online with any other Steam game or mod. Can anyone help me out here? What do I do to fix this.

I have a fresh install of the game.

Here is a picture.
http://img508.imageshack.us/img508/4379/gmodhelp.th.png

Log out.
Delete program files/steam/clientregistery.blob
Log back in.

If that doesn’t work, make a new post. I’m 99% sure it will work.