.avi
 

How do a get a file with a .avi extension to play? I'd prefer to do it in Windows Media Player if possible but QuickTime or any other common media player will do too.

What you need is codecs. VLC (in-built codecs) is wise to have around as a back up player at least. It's ugly but very effective. Get it from the following link.

Also, get GSpot as it will tell you which codec you need to play a particular file. Just open it with GSpot & it will tell you. So then you can get the right codec for wmp.

CODECs list and alternate players for Movies

Gspot
 

So, I've downloaded Gspot and dragged the file to it but it doesn't really tell me all that much since I'm computer illiterate. What do I do after this?

As LOTR said VLC media player does it all for you it plays them all and it's free ......;)