The true and final fix is by deleting these registry entries which are related to Excel 2010:[HKEY_CLASSES_ROOT\FileType\{00020820-0000-0000-C000-000000000046}] [HKEY_CLASSES_ROOT\FileType\{00020821-0000-0000-C000-000000000046}] [HKEY_CLASSES_ROOT\FileType\{00020830-0000-0000-C000-000000000046}] And I can confirm it to you, this problem only occured after I installed windows xp sp4 . But what the relation between Excel 2010 and right clicking/copying files with unknown types?