In case the ScreenCalendar icon appears in your system tray, you should do the following:
right-click on the program icon (in Systray) and choose the Exit option. After that only uninstall the program (Start - All programs - Screen Calendar - Uninstall Screen Calendar).

In case you have already uninstalled the program, I suppose you see html-page that had been formed together with a calendar. So please set (temporary) another html-page as background to turn on Active Desktop automatically.