I'm not exactly sure what it's called, but when you rollover a minimized application in the tastbar the little preview window that should appear has stopped appearing. About a week ago it simply stopped showing up. I run Windows 7 Ultimate 64bit. Thanks
1. Click on Start button and type GPEdit.msc in Instant Search text box, then hit Enter.
2. Navigate to User Configuration -> Administrative Templates -> Start Menu and Taskbar in left pane of Local Group Policy Editor.
3. Locate and double click on Turn off taskbar thumbnails in right pane of Local Group Policy editor.
4. Select Enabled radion button.
5. Click OK to make the change effective.
6. Logoff and logon again or restart computer if needed.
To have the feature activated set the value to Disabled or Not Configured.
This doesn't work on Home Premium. Good thing you have Ultimate.
Another option to do is to download a program called 'Ultimate Windows Tweaker' which is free. This program will let you change many aspects of Windows. Including your problem