You don't have to enter a credit card, and if you do nothing, at the end of the trial it reverts to the free antivirus. You can choose the free antivirus or start a 30-day free trial of the suite. There's a certain amount of upsell when you go to install the free antivirus, but it's much more laid back than, for example, Comodo.
Of course, AVG only makes money if somebody purchases the for-pay security suite.
