It's ONLY the mpeg2 files I render that crash ALL media players - so obviously something is snafu - the only thing I did that I can think of was uninstall Vegas4 so I wonder if mayby a re-install of Vegas 5 would be in order?
If you have another computer and you can get the MPEG2 files to those computers, try playing them there. I bet they will play.
This sounds like a problem outside of Vegas. However, on the off-chance that it is a Vegas problem, the other thing is to make sure you render using one of the standard templates. However, I am betting that you have a problem with conflicts between the jillions of media players that have probably gotten installed over time.