I bought Oblivion + all extensions today and reinstalled it on my computer. The thing is, without me even coming near any controls, my character turns slowly to the right. It wasn't like this before. It's not my mouse that is the problem, because it acts normal in other games. What could it be?

Do you have a controller or joystick attached to the PC? If so disconnect it,

You can also open Oblivion.ini located in My Documents\My Games\Oblivion, find bUse Joystick= and change it to 0.

I don't have a joystick conneted, but I'll try it and see if it fixes things.