Good you found a fix! Are you talking about the MarkC mousefix? If so it does nothing on newer games, there's only a few 20+ years old games it actually fixes anything for (for these old games it is crucial though). Newer games ignores anything mouse related written in the registry, and reads the data directly from the mouse (for the most part).
Other than that, reinstalling Windows can solve a lot of problems built over time that might for some reason impact the sensitivity.