Media Center PC & DVD Upconversion for HDTV

Probably a simple question here.

If I’m running a DVD on a MCE2005 PC and outputting it to an HDTV (at an HD resolution, of course), the software is already acting as an upconversion DVD player, correct?

If so, is there any software that works best in this capacity, or are they all the same since they’re all just decoding an MPEG2 stream and Windows is actually doing the resizing anyway?