Is it in a certain applicaiton? Cause when i click my middle mouse button in Opera i get a new Tab. In firefox i get the window scroll feature, unless i am at the top where the tabs are.
What type of mouse do you have. As mssssee said Intelpoint allow you to change it. So does my Razer software. Might want to check and see if you have mouse software installed. If not try and find it and see if that help at all.