The license key for Avast home edition is good for 15 months. You need a unique key for each machine, but you can have more than one sent to the same email address without a problem.

The program updates its virus db usually everytime I start the computer. However, there have been times when it updates in the middle of a computing session. I guess it looks for updates everytime you boot plus intermittently throughout the day. I don't really know what the schedule is.

The UI is very nice compared to AVG. There are some options not enabled in the Home version, however.

Overall, it's the best antivirus I've ever used. I use it mainly on my PIII 800MHz laptop and performance doesn't seem to have been affected very much at all. You don't have much to lose, so maybe you should just check it out.