If you've used OS X or a UNIX/Linux-based operating system for any amount of time, you get used to being able to hover your mouse cursor over any window or scrolling panel area without having to click on it first. Unfortunately, Windows requires you to click on an area to give it "focus" before the mouse scroll wheel will work. For example, take a simple Windows "Open" dialog box:
It can become annoying having to click on windows or other "scrollable" areas within windows. Sometimes, you end up accidentally clicking on a file or some other object just so that you can use your mouse scroll wheel.
I stumbled across a free, highly useful program called WizMouse that works in 32-bit and 64-bit versions of Windows 2000, XP, Vista, and 7 that solves this very problem. Until Microsoft builds a simple feature into Windows to enable mouse scrolling on mouse hover, give WizMouse a try.