Thank you for that - I will try it
I had a similar problem in WinMe where a video clip at first was displayed by WMP, but later would not. In File Associations I found TWO options of Windows Media Player offered - with different icons. When I investigated further I found that the exe file for WMP v6 was still extant. When I moved all the files with the same date as v6 into a separate folder, the clip would play again, and one of the two options mentioned above had lost its icon.
My conclusion : the clip would play in WMD v9, but not in v6
and the player which was activated was 'randomly chosen' from the two available ones.