Today it happened again to me, for the third time.. i solved the problem before, but forgot it already.
I googled for hour for the solution. But none really work (or not an efficient solution). Fortunately, i recall saving the solution somewhere in my harddisk.
Ok for those having the problems, this is what you can do
1. System restore : i dont think this efficient, you will lost any data or setting of recent software installation.
2. Delete the setting : By browsing to C:\User..\AppData\Local\Microsoft\Windows Sidebar (need to show hidden folder to see this) and delete or setting.ini . this work for some peoples, but not for me.
3. Register some services!: Apparently some services to run sidebar is inactive due to some reason. This is only solution work for me: Open cmd prompt with admin privileges ,type regsvr32 scrrun.dll then regsvr32 jscript.dll and regsvr32 msxml3.dll, restart the sidebar. VOILA!