Actually, there are more causes than that. Search the forums and you'll find the 3 possible causes and solutions for each cause, as this questions has been asked several times already
eg:
Dodgy White Square
Transparent Icons Help
The information comes from the vast Microsoft's Knowledge Base. When you have a problem in Windows, that should be the first thing you need to search in
Anyways, there are 3 causes for the 'problem':
Cause 1:
You have "Active Desktop" enabled
Fix:
Disable "Active Desktop":
* Uncheck "
Use drop shadows for icon labels on the desktop" in
Control Panel > System Properties > Advanced > Performance > Settings
Note that Active Desktop can be enabled by other programs and/or situations or stuff you use
Cause 2:
You have enabled "Lock Web Items on Desktop" feature
Fix:
Disable "Lock Web Items on Desktop":
* Right click on your desktop and choose "
Arrange Icons", there uncheck "
Lock Web Items on Desktop"
Cause 3:
You're using a PNG with a transparancy channel as a background or an animated GIF file.
Fix:
Make sure the background image doesn't have any transparancy channel or isn't an animated GIF. Note that a PNG without any transparant pixels can still have a transparancy channel though and that a GIF file can appear static, but still contain an animation. Use decent graphic programs to check these things.
all the above goes for Windows XP. Similar things might be true for Vista
http://support.microsoft.com/kb/305117/