full screen video zoom

Hi
I'm using a GeForce 6600 GT (VGA, DVI, HDTV).

I have enabled the fonction "full screen video zoom".

First configuration :
- Windows XP
- Latest nvidia driver ForceWare Release 80, Version: 84.21
- standard HP930 monitor connected on vga output
- HDTV panasonic connected via component cable

So I use, "full screen video zoom" by playing my video with Windows
Media Player 10 on the HP monitor and get the full screen result on the
HDTV. Everything work well, the video keep the aspect ratio, when I
play a 4:3 video, the result had black vertical bars on both side.
When I play a 16:9 video, it takes all the screen (keep aspect ratio)
because it's a 1920x1080 HDTV. Good !!!

The problem is with my second configuration ...

Second configuration :
- Same computer same HP monitor, except that I replaced the HDTV
panasonic by a NEC HDTV monitor with a DVI input. So I use the DVI
output of the card to feed it. I configured the NEC monitor as 720p
HDTV and it get the resolution 1280x768p.

I still want to use to "full screen video zoom" feature by playing my
video with Windows Media Player 10 on the HP monitor and get the full
screen result on the HDTV. But this time, it doesn't keep the aspect
ratio of my 4:3 video file, NO vertical black bars. The video look
stretched.

I don't know what is wrong, is it because I use DVI instead of
component output ?

Thanks