AVG almost everyday updates its virus definitions.
So, you need to update it everytime within a week or so.
Things will be easier if you download by the method suggested above-i.e.
Manually checking for updates from AVG software itself.
However, you can always download definitions from the internet. All these definitions have a date system attached, so that you know which one to download.
For eg, if you wanted to download latest AVG definitions as of
April 26th, 2006, here's the link:
Latest AVG updates
Savvy