|As VISHAL CHOHAN advised, it could be a mouse and/or mouse driver problem.|
I have Win 7 x64 Home Premium, the same thing was happening to me, sometimes, mostly in Windows Live Mail and Internet Explorer.
I am using MS bluetooth mouse. Previously I had Microsoft mouse v.7 drivers and I had auto scrolling problem. I installed MS mouse v.8.2 drivers a few days ago because I thought it could be a mouse driver problem.
It may be too early to speak in favor of the new mouse drivers but I am not experiencing the auto-scroll problem any longer.
I will advise if the auto-scroll problem returns.