Killing explorer.exe in Task Manager is not a good idea in Vista. But there is a solution.
In Windows Vista with its standard Start Menu, click on the Start button. Hold down Ctrl+Shift and right-click on any empty area of the menu or on the power/lock buttons at the bottom of the right half of the menu. One of the context menu choices is "Exit Explorer". Choose this and the main Explorer process will cleanly shut itself down.
More details on Aaron Margosis' "Non-Admin" WebLog