Something similar happened to, my system and my house mates.

Have you got the Nimo codec pack installed? I uninstalled this and it worked ok. It could be some kind of codec conflict, ie media player has automatically downloaded one and you have a different one insatlled already or something. (I am not entirely sure what the rules are on overiding codecs)

But the simtoms were very similar, system crashed whenever I tried to open a file with certain encoding and more anoyingly when media player tried to auto preview it.

I would say the best bet is to unistall media player and any codecs you have and then reinstall them in stages, boring I know.