Dim FSO,strDriveName
Dim objNetwork
Dim strLocalDrive, strRemoteShare, strUser, strPassw

Set FSO = WScript.CreateObject("Scripting.FileSystemObject")
Set objNetwork = WScript.CreateObject("WScript.Network")
'strDriveName = "a:"
strDriveName = AvailableDriveLetter
'WScript.Echo "Диск= " & strDriveName
'WScript.Quit
strRemoteShare = "https://webdav.yandex.ru"
strUser = "login"
strPassw = "password"
if FSO.DriveExists(strDriveName) Then
else
    objNetwork.MapNetworkDrive strDriveName, strRemoteShare, True, strUser, strPassw
end if

'-------------------------------------------------
Function AvailableDriveLetter
    With CreateObject("Scripting.FileSystemObject")
        For i = Asc("a") To Asc("z")
            If Not .DriveExists(Chr(i)) Then
                AvailableDriveLetter = Chr(i) & ":"
                Exit For
            End If
        Next
    End With
End Function