if you have license then just enter *.license file and click OK. Then it willl automatically upgrade to AIS.
So its not necessary to uninstall the free av version?
Before a trial version can be run as far as I'm aware you will have to uninstall the free version (as that is the registered version), reboot and install the AIS Trial.
@ 'Prince Winter Flurry'
The trial period is 30 days, I would imaging that you would get a notification near to the time. At that point you would have to either purchase a license to enter, or enter free registration key (you would need that see ~~~ below), or do a clean reinstall of avast free.
~~~
Web site (Off-line) Registration rather than using UI Registration (which wouldn't be available in the AIS UI) see
support.avast knowledgebase article 459 -
http://www.avast.com/registration-free-antivirus.php.