Chris tells it like it is :)
You CAN add the drive letter to your command in that XML file. What that does is tells PowerDVD to launch the video from the specified drive.
However, there's one catch - if you do that, it WILL play automatically. However, Media Center will NOT minimise itself! So you've got to 'Alt-F4' on your Media Center keyboard (assuming you have one) to get to PowerDVD.
I've been meaning to write something small to take care of this problem, but I've been busy. I'll see what I can do on Monday night.
Author of 'EMUCenter' to play arcade classics 'MoreWithMyMusic' to sing with and normalize your Music, 'Yougle Vista' to watch YouTube, DivX & More and 'BlindTimer' to time your poker game.