I used to get that once in awhile with the old versions of Windows. Check that
your screen is set to 16 bit or higher colour mode then find
c:\Windows\ShellIconCache and delete it.

The icon cache file file will have the hidden attribute, so if you can't see it go
into the Tools menu, then Folder Options, then in the View tab set "Show hidden
files and folders"

After a reboot your icons should look better.