Because of the invasive nature of security software, you may, if you run lots of trials, need to use one of the extra uninstaller programs (such as Revo;
http://www.revouninstaller.com/) to make sure that you remove all traces of each install. That should only be necessary if you find that some AV programs won't install properly following the uninstallation of a rival product, (Kapersky appears to be particularly picky about other software) .
Note that I am NOT recommending things like Registry cleaners, which are best avoided. Revo Uninstaller is safe because it creates a Restore point before it starts and you are given the choice as to how deeply it probes. It will identify some Registry entries for attention but I have never had problems using the 'Safe' option.
As with any software, reading the instructions at least twice is the minimum preparation needed.