how can i play avi movies that i have downloaded?

You need an .avi component to play your .avi files in Quicktime.
Download the Intel binary from here:

http://n.ethz.ch/student/naegelic/download/

It's the one at the bottom of the list.

When you have opened it, drag the Xvid_codec-r58 component, and the Aviimporter-r7 component into your HD:Library:Quicktime.

Then restart Quicktime and you should be able to play back your avi files.

Open the file with QuickTime then open the Movie Info window.
You'll see the video codec used in the file in the Format: area.
A Google search for codec name +Mac should help you find a download.

I'm having the same problem as the original poster.

Here is my dilemma. I'm a recent PC user and just got my MBP this past Friday. In an attempt to get all my old files from my old laptop and get my new fancy mac all ready for whatever comes its way, I have stumbled across one file that works just fine on my old computer, video sound and all but I can't get to play on my mac. I have attempted to play the file with both Windows Media Player 9 as well as Quicktime. I have also downloaded the divx codec and flip4mac.

I have also attempted to find the codec used to encode the file by checking in quicktime and all it says in the "Format:" area is ...

, 320 x 240, Millions
8-bit Unsigned Integer, Mono, 11.025 kHz

The video is about 3 years old and was made with a logitech camera but for whatever reason I can't get it to play in either of the two video players. What can I do to get it to play? (Also, in 2 attempts to convert the file to .mov with SUPER converter, it has failed about 60% into the process, so is there a better free converter).

Thank you all ahead of time for helping me with this (and yes, I'll continue to search the endless forums until I get a response or find something).

When QuickTime Movie Info window shows , instead of the video codec actually used in the file it usually means a DivX (or derivative) as the codec.
It also shows a comma when it encounters XviD codec.
Just avoid this issue by not downloading AVI files when you don't know who, or what, made them.

The simplest (and far less complex than the post above re: manually installing codecs) it to download and install the (Free) Perian from Perian.org which handles a host of oddball Windows originated avi files. Start there.

Yep.
Perian works for most oddball AVI codecs.
And it's free.

VLC Media Player also plays .avi files & is also free.

Doesn't help much in playing avi movies in Quicktime though.