This problem is a little difficult to diagnose, but many times I've noticed that on my Windows 7 machine the previews that pop up when hovering over items in my taskbar do not show up.
When hovering over a taskbar entry, normally the application's entry will light up with a color contained within the application's icon. Also, normally, when mousing out of the taskbar, the glow fades away. When the problem I described above happens, that glow is persistently over the last entry of the taskbar I hovered.
I have attached a screencast that clarifies my last paragraph. Hopefully someone can identify why this randomly happens. It reminds me a little bit of that old problem with Windows where tooltips of tray icons would show up behind the taskbar.
Note: Before anyone says this might have to do with the custom visual style I'm using, it doesn't; I just installed this style a few days ago, and before that all I had was the normal Aero interface. The problem still occurred.