Hello, 
There are many type of AVI ranging from divx, xvid, cinepak and etc. When you convert the avi video that was encoded with a specific codec, you will need to have the same codec install on your pc. 
You can download a video inspector software to check the missing codec and perform a search for the codec at 
http://www.free-codecs.com . 
Best Regards; 
Rick 
Softdiv Software