If the icon is missing and you still have the program, you can copy a new icon onto
your quick launch bar by right-clicking the icon and dragging it the toolbar and
clicking copy here.
Download the PowerToy Tweak UI from Microsoft, it has an icon repair utility. http://www.microsoft.com/windowsxp/pro/downloads/powertoys.asp
If the calendar is something you downloaded, it very well could have been spyware.