If your system's icon cache isn't properly maintained, the icons in My Computer, Windows Explorer, the desktop, and in the Quick Launch toolbar may get mixed up. Microsoft has instructions for fixing this at KB 132668: Icons Randomly Change to Different Icons.

In brief, the steps they recommend in increasing order of seriousness are:

1. Press the F5 key to refresh the icons in the current folder.
2. Close and then reopen a window that contains incorrect icons.
3. Restart your computer.
4. Rebuild the ShellIconCache file. Changing your system's color depth automatically rebuilds this file so try changing the color depth and then changing it back. To do this:
   1. Right-click the desktop and select Properties.
   2. On the Settings tab, change the setting in the Colors dropdown and click Apply.
   3. Change the Colors dropdown back to its original value and click OK.
5. Start regedit. Go to this registry key:

   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Explorer

   Add a new string value "Max Cached Icons" and set its value to 2000. (This value should be between 100 and 4096, and defaults to 500. Changing it to 2000 lets your system handle more icons safely.)
6. Delete the ShellIconCache file in the Windows directory.

You don't necessarily need to do all of this. Stop when you get to a step that solves your problem. There have been times when I had to all of these things to get my icons back to normal.