Windows Vista: ActiveMovie Window error message keeps popping up. Help!

ActiveMovie Window error message keeps popping up. Help!

I could simply be reading something on the screen, not clicking on anything, when suddenly an error message pops up. It just started again while writing this. The blue strip reads: "ActiveMovie window: aim.exe. Unable to locate component". The box first says: "This application has failed to start because ad2mpegin.dll was not found. Reinstalling may fix this problem." But reinstalling what?

As soon as I close this, a second box pops up, same title, but now what isn't found is "ad2mcmpgdec." These boxes keep popping up 4 to 6 times before they finally stop. But then it starts again - could be two minutes or a day later.

First I searched my computer for "aim.exe." Nothing comes up, even though I have AIM installed for IM's. Neither do the two .dll files or ActiveMovie window. Thinking this could be an AIM problem, I reinstalled the program, but this hasn't stopped the pop-ups.

What's going on? This is annoying, to say the least, and time-consuming.

I'm running Vista Home and using Firefox 3.5.3. Thanks.