[Imp]

When you say you uninstalled all players that did include windows media player. Its easy enough to uninstall. Once you find the problem putting a clean install back should not cause it to reoccur.
Have you tried checking what the files are associated with, click on a sound file & the intro video if its a seperate file & windows should select your media player if still have one. Or can right click & select properties to find program associated with or at least I am set up that way. Have a few aftermarket if that does not work stick file associations in windows help.
Still sounds suspicious to me if movie pops up randomly not at start something is controlling that, try running a decent process monitor & see what changes between version 3 & 3.5. There is a free one I think surprisingly called Process monitor that can be set to record a log has filters etc. Compare find what extra things running & google it.
Also as Andy mentioned I assume you have checked sound video drivers are up to date this often solves problems.
Edit
Also forget whats called but checked your version of directX or whatever its called. If you have installed an old game bad programing means it may have installed an old version.