I had seen this error message on another Vista box previously, but at that time it definitely was related to a wrongly installed HDA driver for the onboard sound. This time sound was working, and some videos did actually play, so I was peeved when I got this error message while trying to watch recorded Web casts:
Yesterday, my solution was to boot XP - but leaving Vista just for watching Web casts, well, that is ridiculous. So I set out for a search on what other media players are capable of viewing WMV files. In the end (the third one I tried) I settled for the free DivX Player which nicely views all the recordings Windows Media Player 11 balked at.
Other casualties to report: the Sony Ericsson tools PC Suite as well as Update Service crash and burn on installation (the first one with MDAC, the other one with an already well-hated Java Exception, this time however on x86 and a different index).