Drop the avi onto the new comp button (its in the library window, just below it… looks like a filmstrip.)
Then with that comp on the screen, click Effects - > Color Key.
Use the eyedropped to select a base color then use the tolerance and feather to achieve the best result/
Gl.
EDIT: It’s also worth noting that if you need to re-export it as an AVI, that’s not gonna work. AVI files do not natively support an alpha channel. So you’d need to re-export as a quicktime movie or something else that supports the alpha.
Keep in mind too that white is a really bad key color, you typically want green or blue, so if there’s any white in the middle bit, take care that it isn’t getting knocked out too. (Like the whites of someone’s eyes, that might look goofy.)