This may or may not work for you but it is worth a shot.
"It happens to me few days ago - the option become greyed. The most strange was that it was accessible previously. The only thing what I needed to do to "ungrey" this it was changing some options in taskbar, so try this: right click on the tasbkar > Properties > change something (for example uncheck > check Lock the taskbar ) so the Apply button become available > press Apply button > press OK > open one more time taskbar's properties > checkbox for use Use Aero Peek... should be accessible again.
If it doesn't work, try this: right click on Computer icon on your desktop > Properties > from the left pane: Advanced system settings > Advanced tab > press Settings... button from Performance sections > check Enable Aero Peek" from http://answers.microsoft.com/en-us/windows/forum/windows_7-desktop/i-cant-seem-to-turn-on-aero-peek-when-i-click-on/0e242d49-fa33-4919-bb9f-6c327ba75fd7