The scroll wheel is probably just dirty. A tip that I picked up in another thread was to turn it upside rub the scroll wheel around in all directions a few times on a clean sheet of paper
(does that make sense?). I did that, except on my mouse pad and it worked. Only the thing was clogged up again in no time. I have since bought another mouse that doesn't suck. There are a number of threads regarding people's favorite mice.
My advice is, just keep cleaning it as needed or get a better mouse.