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.

For me this was solved by copying (or rather screenshotting) all settings in "personalization", then activating the "Windows 7" theme, applying all settings again and then saving that as a theme.