I just got a new computer and transfered all my files, but I can’t play AVI files. RealPlayer just plays the sound and WMP won’t play them at all. Do I need to download a special codec?
Normal AVI files should play right out of the box, but DivX-encoded files often have an AVI extension and require a separate codec. You can grab the codec at www.divx.com (click ‘Free Download’). Don’t listen to the hype on that site about the DivX player – I believe you can just play them in WMP once you have the codec.
They might not be DivX, though – they could be needing one of those other newfangled codecs that I can’t keep up with. But DivX is worth a try, and you’ll probably need it eventually if you don’t already have it. If that doesn’t work, I think there are software packages out there that can identify the codec a particular file needs. Maybe someone else will chime in.
I was having problems recently with MS Media Player not playing avi files. Someone here recommended VLC Media Player which works quite well, no additional codecs to download…
I just remembered the name of a software package to figure out which codecs you need.
The rather unfortunately-named GSpot. Just be glad you weren’t the one who had to google it 
So that’s something else to try if Hail Ants’s suggestion doesn’t work for whatever reason. I’ve never used it before, though, so I can’t make any guarantees.
.avi is just a container file, not really an audio or video format. You can put all kinds of differently encoded audio and video into it, so if the video isn’t playing, it’s almost certainly a codec problem.
VLC is a nice simple powerful program. It supports most popular formats, plus I like the keyboard shortcuts, like “f” for fullscreen toggle, Control or Alt and the left/right arrow keys for fast forward/rewind, and space for pause/resume. Plus you can customize all the controls.
Rather than download new codecs every time you come across a differently coded avi file, try downloading ffdshow. It’s a codec library that will allow you to play just about any video you’re likely to come across on the internet (except .rm and .mov, which require different software).