Hope it helps.
Problem In Windows 7, when you hover your mouse on taskbar icons, it will display small thumbnail preview of opened window on your taskbar, but if you are not comfortable with taskbar thumbnail previews feature, you can disable it. Solution
Open Local Group Policy Editor. To open Registry Editor, type in "gpedit.msc"in Start menu search box and hit enter to validate.
Navigate to "User Configuration->Administrative Templates->Start Menu and Taskbar" in the left pane of Local Group Policy Editor.
In the right pane, find and double click "Turn off Taskbar Thumbnails".
To disable taskbar thumbnails preview, check "Enable" and click "Apply". To enable taskbar thumbnails preview, check "Disable" and click "Apply".
Click OK button to finish setting and restart computer to check your taskbar for thumbnails preview.
Taskbar Thumbnail Preview only works when you have turned on Aero in Windows 7.