I never had this problem before, but now when I download UT3, the mouse keeps moving to the left. Every time I try to mess with a slider, it starts moving continuously to the bottom of the bar, and when I get in-game my character just keeps turning.
Has anyone else had this bug before? I Googled it, but I found one response pertinent to the problem, and their solution was uninstall and reinstall the game. I did that, but still nothing.

Yeah, either having a pad connected (such as the Xbox360 controller for PCs) or a 3D mouse (with extra toggles for CAD drawings, etc.) can cause that. Unplug all external controllers except for your mouse.
If you do have a 3D mouse, you may want to consider using a different mouse without the extra controls, or disabling the related HIDs in your hardware profile.

http://www.youtube.com/watch?v=nUworKXBzdE instructional video

If its a joystick, check the throttle, thats the one that gives me trouble the most

Just to drive the point home, let me say what others have already said: this problem usually stems from a gamepad or a joystick that's connected to the machine. Unplug it, end of problem.

lol
Damn it, that song always reminds me of Forrest Gump for some reason.. Can't remember if it was on the soundtrack... RUN FORREST, RUN!