I have only heard about this in relation to the avast pro trial version. Instead of the 60 trial period, if the program is uninstalled then reinstalled the 60 day period is invalidated and you get an extension of 7 days(I think). This is an anti-piracy measure.

I don’t think it happens like that to the Home version, but it too has a 60 day trial after which time the program has to be registered (free) and a home registration key is emailed to you. This is valid for 14 months, then it needs to be renewed, again free.

Is there any possibility that somehow you installed the avast pro trial? What does the ‘About avast!..’ say (right click avast icon).