I'm not sure of this (I can't fire up an 8.6 just now), but if you go to the quicktime control panel, I think you can reset its prefs to make Quicktime the default for all media types.

btw--if you drag the file from the download folder onto the quicktime player icon, that should override the setting for RealPlayer.

I sure hated the way RealPlayer would mess up a system by resetting all the mappings. Grrrr.