Duncan,

Assuming you are using KiX 4.1+
Try using the CDbl() and FormatNumber() functions:
code:
$objWMI = GetObject("winmgmts:{impersonationlevel=impersonate}!//" + @WKSTA)

$colDisks = $objWMI.ExecQuery("select * from win32_logicalDisk")
For Each $objDisk In $colDisks
if $objdisk.mediatype = 12
$name = $objdisk.name
$volsize = $objDisk.size
$Kb = CDbl($volsize) / 1024
$Mb = CDbl($Kb) / 1024
? "Name is: " + $name
? "Volume size in bytes is: " + $volsize
? "Volume size in kb is: " + FormatNumber($Kb,1)
? "Volume size in Mb is: " + FormatNumber($Mb,1)
endif
Next
$objWMI = 0

I just tested this code, works on XP

-Erik