Probably this will work:
 Code:
Dim $wdStartupTemplatesPath = 7
Dim $wdWorkgroupTemplatesPath = 3

$objWord = CreateObject("Word.Application")
$objOptions = $objWord.Options

$objOptions.DefaultFilePath($wdStartupTemplatesPath).Value = "C:\Brand Templates\startup"
$objOptions.DefaultFilePath($wdWorkgroupTemplatesPath).Value = "C:\Brand Templates"
$objWord.Quit