Question: This morning I had to restart my windows 7 computer due to a security patch. As it was restarting, all the icons on the desktop appeared, then turned white and then the original images started reappearing. This is a common phenomenon after a restart and I've seen it other times (such as when I minimize all my windows). Can someone explain how this happens?

Answer: 
Hate to say it, but these answers are all wrong. :)

To stop icons from refreshing, hit the START button, and in the box at the bottom, enter "msconfig". Click on msconfig.exe (which launches System Configuration), click on the "services" tab. Scroll down the list and deactivate the "Server" service. Click "ok" and reboot computer. Voila - no refreshing of icons anymore.

Needless to say that the "server" service has a purpose (which is, I believe, to support file, print, and named-pipe sharing over the network), so you need to decide for yourself if you want to deactivate it. But this is the one that refreshes the desktop and makes booting up seem so incredibly slow.
It's the intial load of your desktop by the OS, then a refresh. There could have been a process running in the background, and then terminated, which could have caused a refresh.