Most probably, you have an AVI with a CODEC, that is either installed on your system (many players have their own set of CODEC's, for instances where the necessary one is not installed on the system), or a CODEC, that PrPro cannot edit with. See this article: http://forums.adobe.com/thread/440037?tstart=0
Report back with the CODEC inside that AVI "wrapper," and someone can either point you to the proper CODEC, or tell you how to convert that file to a CODEC, that PrPro can use.
My first guess would be DivX/Xvid, or next, MJPEG.
Good luck,
Hunt