Defragments volume D on a computer. If you modify this script to defragment a different volume (such as X), note that your WQL query must specify the drive letter followed by a colon and then followed by two slashes. Thus volume X would be listed as X:\\.

Visual Basic
strComputer = "."
Set objWMIService = GetObject("winmgmts:" _
    & "{impersonationLevel=impersonate}!\\" & strComputer & "\root\cimv2")

Set colVolumes = objWMIService.ExecQuery _
    ("Select * from Win32_Volume Where Name = 'D:\\'")

For Each objVolume in colVolumes
     errResult = objVolume.Defrag()