After new software has been installed, your browser tries to open downloaded files with the wrong application.
Sometimes Mac OS X gets confused about which program should handle the various kinds of media files and streams. You can take several steps to associate Real content with RealPlayer for Mac OS X.
Find one of the downloaded files in the Finder.
Hold the Control key down and click on the file.
Select Get Info from the file's menu.
In the Open With section of the file's Info window, choose RealPlayer as the application.
Click Change All.
In the Media Types pane of RealPlayer's Preferences, be sure that all of the media types listed are checked. (This will not affect content intended for other media players.)
If QuickTime is trying unsuccessfully to play RealPlayer streams:
Open up the QuickTime pane of System Preferences.
In the QuickTime Plug-in MIME settings dialog, turn off Streaming (RTSP and SDP).