I have never paid for Anti virus and never had any problems using one of the freebies, that said I'm not into trawling the seamier side of the internet or mass P2P downloading. There are currently loads of free AV progs, many of them submitted to VirusBulletin and other independent testers with good results.
As to sequence of events download the installation file first, disconnect from the internet, install the new package reconnect and update. My current favourite is avast! but they're all pretty good.