hmm.. How big is the file??
Also even though your max php file size 100megs, you also have a php timeout setting. So if your upload is too long, it will just fail.
Also below the setting to turn on multimedia files, make sure .avi is listed in there.
Try uploading a smaller avi, something that you know isn't too big. I would try to stay as small as you can. Don't try to upload a 50meg file to test...
Also, make sure your uploads folder attributes are set to chmod 777... Also I think the data folder (along with sub directories) need to be 777 as well, if my memory serves me right.