Ichinisan is talking about our laptop and I've been trying to trace the problem for a long time. GSpot was one of the first utilities used. Last time, it was me trying to get MJPEG video to work (Should work with Windows "out of the box" according to other people). It was a video recorded on a Canon digital camera. It would say that everything was correct unitl it started rendering then it would "fail" about a third of the way through. It would fail immediately if attempted again without a system restart.
Basically, I had to try 4 or 5 different trial versions of commercial MJPEG products before I found one which would work with TMPGEnc to side-step that problem, but it has manifested with various codecs including ones that are supposed to me supported on a fresh Windows/Media Player installation. This isn't the first time I've had such "unfixable" problems either... My friend's Hollywood Magic Xcard borked his MPEG playback for all applications. My Hollywood Plus did something screwy like that to me once with Windows 98. Even after uninstalling and removing any and all trace of the drivers, MPEG playback still didn't work and there is no "codec" to install to fix that sort of thing. Installing and removing again would not fix the problem either.