The MKV format, although never originally intended to share stuff online because it's large and really high quality, has now become standard. Especially now with Blu-Ray becoming a standard. But some of us are happy storing AVI for our own use especially if we can have it in decent-enough quality (MKV files are usually 1GB or more for, say, a film).
Google is full of tools and utilities that allow MKV to AVI conversion. Many of them are shareware. You download them free but then you have to cough up $29.99.
I have found the FREE tool, All2AVI ( http://sourceforge.net/projects/alltoavi/ ) does the job and does it superbly. It's fast, free, and the conversion from MKV and other formats to AVI happens pretty efficiently and reliably.
If you're on OSX, you can use VisualHub or Submerge, but both of them are a bit unreliable. VisualHub in particular is a fantastic way to do any conversion on Mac OSX, but if it collapses with a cryptic "Sorry, could not convert for some reason", then you may wish to use AlltoAVI inside an XP virtual machine.