Hopefully this gets you a little closer to what your looking for:

Code:

break on

$WMI = GetObject("winmgmts:\\"+@WKSTA+"\root\cimv2")

$DiskSet = $WMI.ExecQuery("SELECT * FROM WIN32_LOGICALDISK WHERE DriveType=3")

$TotalSpace = 0.0
$FreeSpace = 0.0

For Each $Disk in $DiskSet
 $FreeSpace = $FreeSpace + $Disk.FreeSpace
 $TotalSpace = $TotalSpace + $Disk.Size
Next

$sizeingigs = $totalspace / 1073741824

? $sizeingigs