I had this happen on one on my users new win7 p.c's...
this worked for us.
Rebuilding the Windows 7 Icon Cache.
Follow these directions:
1. Close all folder windows
that are currently open.
2. Launch Task Manager using the CTRL+SHIFT+ESC key sequence, or by running taskmgr.exe
3. In the Process tab, right-click on the Explorer.exe process and select End Process
4. Click the End process button when asked for confirmation.
5. From the File menu of Task Manager, select New Task (Run…)
6. Type CMD.EXE, and click OK
7. In the Command Prompt window, type the commands one by one and press ENTER after each command:
CD /d %userprofile%\AppData\Local
DEL IconCache.db /a
8. In Task Manager, click File, select New Task (Run…)
9. Type EXPLORER.EXE, and click OK.
Your icons should be as good as new!
google it and you will find other guides to help you.