As far as we know, you cannot convert single .ifo file to AVI video, which means direct ifo to avi conversion is not possible. But in case that you really need to, it should be possible to convert whole DVD media (.ifo, .vob etc.) to a single video format (like the AVI), with some dedicated DVD Ripper software specially designed for this purpose. What this will do, is that it grabs and convert video data and saves them in one of the multimedia files, often compressed. The resulting quality might be lower however.
Open IFO file Open AVI fileA video ripping tool for Mac
A DVD converting and ripping tool
A DVD ripping software package
Older video converter from Wondershare