It isn't correct that avast needs to be updated before it is switched on, that wouldn't be possible for a fresh install as the VPS signatures are contained within the latest program setup. Once installed when you next connect avast will check for updates, it isn't a requirement.
I switched on the computer and found that Avast had for some reason had to be up-dated before my anti virus was switched on
What was the exact message ?
What is your firewall ?
Does it allow avast.setup internet access ?