You should have a spare copy of sidebar.exe in the Windows\winsxs folder. You'll need to search for Sidebar.exe to find it, as the folder names use GUIDs. Then copy it into Program Files\Windows Sidebar and see if that fixes things.
the former shows you how to take permission of individual files,
while the later gives you away to just right click on any file and take permission (what i use). Its a non-harmful script that you can undo.