First - check that you haven't any viruses/malware/trojans etc., using the
frequently posted recommendations on these news groups!!

Only then - make sure that you have your Folder Options set so that you can
view System files and folders, and Hidden Files.
Now - Open Windows Explorer and go to c:\Windows - look there for the file
'ShellIconCache'.
DELETE IT
reboot, and windows will rebuild the file.

If that doesn't work, then you may need to increase the size of the cache -
download and install the IconCache Fix from
http://www.aumha.org/reg1.htm

But do check for viruses first!!

Mart