Yes, that's what I said. PowerPoint opens with the file association method.
If you use the code from Shawn though it will open correctly as a show.

Dim $FileName, $PowerPoint, $Presentation, $Nul
$FileName = '\\server01\presentations\WELCOM~1.PPS'
$PowerPoint = CreateObject('Powerpoint.Application')
$Presentation = $PowerPoint.Presentations.Open($FileName,,,0)
$Nul = $Presentation.SlideShowSettings.Run
Exit 0