To answer your question, here are the only two choices:
Comodo Internet Security (very good and recommended):
http://www.comodo.com/home/internet-security/free-internet-security.phpAd-Aware Free Internet Security:
http://download.cnet.com/Ad-Aware-Free-Internet-Security/3000-8022_4-10045910.htmlAs for whether you should have premium protection...YES! It is the best idea to do. It is so worth it, especially with the amount of protection it serves. I recommend only a couple different ones those.
For the program I most recommend is Kaspersky Internet Security. It yields the highest results in antivirus testing groups, and is one of the most trusted. If you would like to know more, please click on the Kaspersky logo in my signature and it will take you to the appropriate product choice page so you can understand all of the different products. We're truly lucky Kaspersky exists as one of the best, because it beats most other products by miles.