If the video's are downloaded from the internet, then there's a good chance
you need the DiVX codec or one of it's variants. You can get the main DiVX
codec from www.divx.com
There are several variants of this codec, such as XViD so you're probably
advised to search on google for a program called G-Spot (I don't have a link
handy). This usefull little program can tell you what codecs you need to
play any particular AVI.
I don't recommend you install one of the many codec packs on the internet.
It's a bad idea to install too many video/audio codecs at a time as it has
been known to cause problems, and some of these packs contain codecs which
may be commercial and are being distributed illegally.
