Visit here: http://free.grisoft.com/ww.download-update to download the updates of AVG Free 8.0
There are always 4 files available to be downloaded, but you should download them according to the last date you updated your AVG Free 8.0
If the last date is after the date specified in the site, you don't need to download.
On the contrary, if the last date is before the date specified in the site, you should download the files.
AVI and IAVI are both files that you should download at the same time so that your AVG Free 8.0 virus definitions are up-to-date.