Download and install the latest version of quicktime player from apple.com . Then go onto limewire and search for quicktime serial numbers to upgrade to pro. You will find many! Make sure it's a serial number for the version of quicktime you've installed. Register quicktime with the serial number and wulla you can play .avi movies. Don't worry about mac figuring out it's pirated! Mac makes it's money off of hardware and not software. they want as many mac users as possible so they don't cross check serial numbers! This is true for there operating system also. They don't even require serial numbers on thier latest operating systems. Just install and register your name.

Also on many miss named movies you can simply rename it with .mpeg at the end of the title and quick time will convert it. Same with music files simply try renaming it with .mp3 at end.

P.S. check out www.mac.com! Its a free service for mac users only. e-mail, web pages, personel photo galeries etc.