Lucky me (and you if you are reading this): I seem to have found the problem. We all know the UAC of windows vista. Some of us hate it, some of us don't care. I was one of those who didn't really care, though because it's quite annoying to press four buttons everytime you are doing something, I switched it off. Now normally one would think that everything now starts in administrator mode (and it should). But apparantly, there's still a difference... It seems that my games aren't run in administrator mode and that's why there's the mouse lag.
If you just go to the properties of the *.exe of you game and switch on "Run as administrator" in the "Compatibility" tab, then everything works as smooth as ever! Hooray!
If you have this problem and UAC is on with you (or you don't have a clue what UAC for vista is) then just try running the game as administrator in the same fashion I just explained above.
Enjoy fragging pixels again!