This occurs when the icon does not contain the proper size that you're trying to display them at. Windows *.ico files typically contain a series of different sizes, i.e. 256x256, 128x128, 72x72, 48x48, 32x32, 16x16, etc, from largest to smallest.
When the display size is set larger than the maximum resolution icon provided, Vista draws a faint border around the icon, but doesn't attempt to upscale it. If you really need to fix this, you can add a high resolution (256x256px) icon using a resource editor.
Resource Hacker and tutorial:
http://www.angusj.com/resourcehacker/http://www.askvg.com/tutorial-all-about-re...brief-tutorial/Icons:
http://www.iconspedia.com/icon/google-calendar--744.htmlhttp://www.ipodtouchfans.com/forums/showthread.php?t=152065