How do I enable the Windows 7 taskbar hover preview?

Right click on Computer » Properties » Advanced system settings » Advanced tab » Performance settings. Here, uncheck/check 'enable desktop composition'.

This will disable/enable preview of windows on the taskbar.

None of the Aero Peek settings or registry edits work.

WARNING: this will result in many other changes and cannot be easily undone.

You need to enable Aero

Right click on your desktop -> personalize

Edit : Or you have recently installed an application that dosen't suport Aero, so you can't reenable it.