SOLVED: I need to disable Flip 3D on my mouse

For ages now, I've used the middle click on my mouse to do things like open new tabs, but for some strange reason, it's suddenly been mapped to the Flip 3D feature. How do get my mouse back to the way it was?

Mouse Settings under Control Panel...?

There's nothing about mapping mouse keys to different actions in Mouse Settings. I've checked multiple times.

Check in the Start menu for a folder with the name of your mouse's manufacturer on it (eg: Logitech, Microsoft). See if you can find more specialized menus for the mouse in there; usually that's where you can redefine what button does what. (Personally I mapped all the buttons to generic mouse buttons, and am slightly annoyed that Source can only read the first five...)

Oh wow, I just did that and it worked! Thanks so much!