Hi Mark,
updating of avast! virus database and avast! itself is simple automatic task. If you use default settings and you are connected to internet then:
1. avast! checks after connection and every 4 hours for new version of virus database - it needs about 100 bytes of trafic
2. if there is the new incremental file (1 to 100 KB), it's downloaded, the virus database file is updated and reloded to avast!
You can check for new update and download it by clicking to the flash icon. The similar technology is used for program update too. But program update needs a computer restart.
Maybe it helps Eduard