http://free.avg.com/gb-en/faq.tpl-crp.num-5169
Once your AVG Trial version expires, you will receive the following message:
You have lost your AVG protection! Your AVG protection has expired.
You have two options to resume AVG protection:
Installing AVG Free
Purchase a license to continue Advanced Protection
When you are notified that trial has expired, click Express Purchase, or Buy now to purchase and continue using the advanced protection.