Trial versions of anti-virus products are good for testing products but they do not receive updates, so any new virus that is introduced after the trialversion was released will have total access to your PC.
According to Daily Tech, you can get around that pesky requirement to have Windows XP or Windows 2000 already installed on your PC by instead using the upgrade DVD to first install a 30-day trialversion of Vista --that's done simply by not entering the product key when prompted to do so during the installation.