If you open System Restore and try to choose a restore point but the calendar is missing, it's probably the Hypertext Markup Language Component associatiion (.htc) that is missing or corrupt.
Cut and paste everyting below into notepad, save it as a .reg file, and then double-click on it and let it put the information into the registry:
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\.htc]
"Content Type"="text/x-component"
@="htcfile"
[HKEY_CLASSES_ROOT\MIME\Database\Content Type\text/x-component]
"CLSID"="{3050f4f8-98b5-11cf-bb82-00aa00bdce0b}"
"Extensi on"=".htc"
HKEY_CLASSES_ROOT\CLSID\{3050f4f8-98b5-11cf-bb 82-00aa00bdce0b}]
@="Microsoft Html Component"
[HKEY_CLASSES_ROOT\CLSID\{3050f4f8-98b5-11cf-bb82-00aa00bdce0b}\InProcServer32]
@="C:\\WINDOWS\\System32\\mshtml.dll"
"ThreadingModel"="A partment""
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\.htc]
"Content Type"="text/x-component"
@="htcfile