tohle by mělo fungovat:

Set WshShell = WScript.CreateObject("WScript.Shell")

Dim fso, d, dc
Set fso = CreateObject("Scripting.FileSystemObject")
Set dc = fso.Drives
For Each d in dc
If d.DriveType = 2 Then
Return = WshShell.Run("defrag " & d, 1, TRUE)
End If
Next

Set WshShell = Nothing