I had the same problem with Aero Peek being disabled and greyed out (cannot select enable or disable).

This was the fix for me:

"Desktop Window Manager Session Manager" System Service was apparently not enabled or running/started. It was possibly set to disabled or manual and would not load when the system booted up and loaded my Windows 7 Home Premium 64bit.

I found this out running the troubleshooter program and it automatically fixed the problem so I'm not sure what exactly changed to fix the problem because I didn't make the change myself, windows 7 did it automatically through the trouble shooter, but my guess is that it was somehow disabled in the services.msc run command tool.

How do I know that it is fixed? The troubleshooter said something along the lines of: "Service Started. Problem Fixed."

I right clicked the "desktop" and went to "personalize" to load the "themes" and went to the "Aero Themes" and selected the first one called "Windows 7" in order to enable the default settings which have "Aero Peek" and after I selected that theme all the windows changed to the blue color with what looked like "Transparency" enabled.

I looked to see if Aero was available in the "system properties" - "appearance settings" and it was NOT there in the list of appearance options.

I right clicked the taskbar and went to properties and the enable/disable check box was still greyed out and wouldn't work.

So then it occurred to me. If the fix was done, the computer system probably needed to be restarted to activate all the options that were missing and/or disabled. After a restart, sure enough "Aero Peek" was found in the "System Properties" / "Appearance Settings Options List" and the "Taskbar Properties" "Aero Peek" enable/disable option was working with "Aero Peek" enabled (the checkbox was checked).

I went to themes and loaded my primary customized theme with a glossy black taskbar and top window border, and dark charcoal background of a window logo/symbol and it loaded perfectly and the transparency option was available in the "color settings" and I was able to choose different colors which wasn't possible when the service was turned off and was one of the source of problems that caused me to go search online about what was causing the disabled and broken appearance settings.

So there you have it. A potential fix for you. Either use the troubleshooter or load the services in the run command or search "services" in the search box in the start menu and make sure "Desktop Window Manager Session Manager" or "Desktop Window Manager" is set to automatic and restart the computer if this is the case.

When the problem occurred it seemed to be after I installed some new drivers for the graphics chipset and/or when I hooked up a tv to the computer via HDMI. It was around that time that I noticed the windows and taskbar were blue and my desktop background image was changed. I remember that when I installed the new chipset drivers it said something about "Aero Peek" in the installation menu tool and provided an option to deselect a check box next to that option. My best guess is that this driver is what caused the problem and shut down the system service or disabled it all together.

The trouble shooter said the service was called "Desktop Window Manager". However, when I looked in the services the only service close to that name was "Desktop Window Manager Session Manager" and it was "running" or "started" and set to automatic (after running the troubleshooter) so I am guessing it was the right service that needed to be changed.

This should be the fix for most people out there with this problem.