This change the network settings to 100/Half. The setting, 3 in this case, was found by manually changing it and then checking
the registry entry.

It doesn't use WMI but might be of some use..

************************************************************************************************

On Error Resume Next
Set shell = CreateObject("wscript.shell")
Dim strRegKey, strregVal, strDone
strDone = "No"
for i = 1 to 20
  strRegVal = ""
  strRegKey =
"HKLM\System\CurrentControlSet\Control\Class\{4d36E972-E325-11CE-BFC1-08002­bE10318}\000"
& i
  strRegVal = shell.RegRead(strRegKey & "\DriverDesc")
  if strRegVal = "" then
     i = 1000
  else
     Select Case strRegVal
        Case "Intel(R) PRO/100 VM Network Connection"
           shell.RegWrite strRegKey & "\SpeedDuplex", 3
           strDone = "Yes"
        Case "Intel(R) PRO/100 VE Network Connection"
           shell.RegWrite strRegKey & "\SpeedDuplex", 3
           strDone = "Yes"
        Case "Intel(R) PRO/1000 MT Network Connection"
           shell.RegWrite strRegKey & "\SpeedDuplex", 3
           strDone = "Yes"
     End Select
  end if
next
if strDone = "Yes" then
   msgbox "Your network adapter has been set to 100 Mbps Half duplex."
else
   msgbox "Unable to set network adapter speed."
end if

Note: "HKLM\System\CurrentControlSet\Control\Class\{4d36E972-E325-11CE-BFC1-08002­bE10318}\000" is the regisrty entry for a given perticuler network adapter.

Please change the entry according to your network adaper details.