See if this helps...

Code:
 
Function GetFreeSpace($Computer,$Drive)
   Dim $oComputer,$cItems,$oItem
   $oComputer = GetObject("winmgmts:\\" + $Computer + "\root\cimv2")
   $cItems = $oComputer.ExecQuery("Select * from Win32_LogicalDisk where DeviceID='" + $Drive + ":'")
   For Each $oItem In $cItems
      $GetFreeSpace = 0.0009765625 * $oItem.FreeSpace
   Next
EndFunction