To extend my example and wrap it in a function...

Code:

fnChangeDefaultIcon('.kix','%WINDIR%\system32\SHELL32.dll,21')
@ERROR " | " @SERROR ?
 
Function fnChangeDefaultIcon($sExtension,$sIcon)
   Dim $sFileType,$rc
   $sFileType = ReadValue('HKCR\' + $sExtension,'')
   $rc = WriteValue('HKCR\' + $sFileType + '\DefaultIcon', '', $sIcon, 'REG_SZ')
   Exit @ERROR
EndFunction