I find Microsoft Windows Vista Media Center is great in so many ways, but nothing's perfect (more's the pity).
One thing which has been irritating me for a while is how it was using the "shrink-to-fit" method of sizing local video playback instead of "expand-to-fit". This results in any wide-screen videos losing the left and right edges, as I only have a 4:3 monitor (shame on me). I believe it'd be causing equal problems with non-wide-screen videos if I had a wide-screen monitor, though.
Until a few days ago, I'd been using ffdshow's capable suite of options to force output into 4:3 (while preserving the aspect ratio, i.e. adding evil black bars). This really eats CPU, even on the fastest resize mode, increasing usage by approximately 30%, which was actually causing my AMD64 3200+ (2.0GHz) to run out of cycles on some videos.
What I found a few days ago, though, was the real solution: time to start the Registry Editor, folks. Locate the following key:
Under this key there are a large set of options, most of which you don't want to touch. In this case, however, one particular one is crucial - ZoomMode_Video. Set that to "0". Done.
Footnote: There are 3 other ZoomMode_ options (ATSC, DVD, TV) and all 3 were already set to "0" here. ZoomMode_Video, however, was set to "1". I'd be very interested to hear how many, if any, other people have any of these ZoomMode_ options not set to "0" and which ones.