In my case it is not downloaded movies - it is children's TV shows recorded on a TiVo, MCE or BeyondTV, and then converted using AutoGK to, I believe, xvid. It doesn't appear to be one or two files - it is the whole lot of them.
Could there be something in my setup that is making this problem worse.
But I have never had the slightest problem with these files in the past. I have had them for years, some are at least 3 years old, on multiple PCs running different versions of XP Media Player and they work fine. It is just in Vista that I have problems.