You can change the default for which program opens particular formats using the pc exchange control panel. Go to your Apple menu>Control panels>File Exchange & select the format (listed alphabetically) & change the default app that opens it. I think from memory it's best if you open the advanced window. So if you want QuickTime to be the default then select the format on the list & choose QT as the program to open it. I can't remember whether this has automatic effect or works after the next time you reboot. BTW if you see RealPlayer listed as a program to play certain formats, then it 'might' be an idea to remove RP from the pc exchange settings. I don't know.