Thanks for the information!
I added the UDF's and attempted to use the WriteValue function. But when I run in debug mode I see that it identifies the MTU value in 2 of 4 keys and I get an error about a missing comma? It scrolls extremely fast so I might have missed something.



Code:
  

Break On
Dim $BaseKey, $Key, $Value
$BaseKey = 'HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces'
For each $Key in ArrayEnumKey($BaseKey)
  If AScan(ArrayEnumValue($BaseKey+'\'+$Key),'MTU')<>-1
   'Value ['+ReadValue($BaseKey+'\'+$Key,'MTU')+'] in Key ['+$Key+']' ?
  EndIf
Next  

WriteValue( $BaseKey , $Key , $Value "MTU", "1400", "REG_DWORD")
If @ERROR = 0
   ? "Value written to the registry"
Endif

;UDF Function ArrayEnumKey

function arrayenumkey($regsubkey)
    dim $retcode, $subkeycounter, $currentsubkey, $subkeyarray
    
    if not keyexist($regsubkey)
        $arrayenumkey=''
        return
    endif
    
    $subkeycounter=0
    do
        $currentsubkey=enumkey($regsubkey,$subkeycounter)
        if $currentsubkey<>259 and @ERROR=0
            redim preserve $subkeyarray[$subkeycounter]
            $subkeyarray[$subkeycounter]=$currentsubkey
            $subkeycounter=$subkeycounter+1
        endif
    until $currentsubkey=259 or @ERROR
    
    $arrayenumkey=$subkeyarray
endfunction

;UDF Function ArrayEnumValue

function arrayenumvalue($regsubkey)
    dim $retcode, $valuecounter, $currentvalue, $valuearray
    
    if not keyexist($regsubkey)
        $arrayenumvalue=''
        return
    endif
    
    $valuecounter=0
    do
        $currentvalue=enumvalue($regsubkey,$valuecounter)
        if $currentvalue<>259 and @ERROR=0
            redim preserve $valuearray[$valuecounter]
            $valuearray[$valuecounter]=$currentvalue
            $valuecounter=$valuecounter+1
        endif
    until $currentvalue=259 or @ERROR
    
    $arrayenumvalue=$valuearray
endfunction