You'll find VLC more reliable for previewing & watching in general:
http://www.videolan.org/vlc/ (choose nearest mirror site) It has in-built codecs.
Yep your problem sounds like a codec plug-in issue. There's many out there. There's also DivX, XviD, 3ivx, etc.
As far as T3 goes, well there has been a conspiracy theory. lol