UB has provided the essential part of your answer. It's only version 7.1 of AVG Free which will cease to be supported in January. Version 7.5 of the program is already available and you can download it now. (It's still free and not time-restricted):
http://free.grisoft.com/
There is absolutely no reason for you to consider paying for any security software. VHG has correctly identified the most popular firewall, with AB members, as Zone Alarm Free:
http://www.zonelabs.com/store/content/company/ products/znalm/freeDownload.jsp?dc=12bms&ctry= US&lang=en
Those two programs are perfectly capable of dealing with any real security risks to your PC. Ad-Aware will deal with spyware (which, despite its name, isn't an actual security risk - it just slows your PC down). CWShredder will help prevent anyone who has physical access to your PC (rather than via the internet) from accessing deleted information.
The free editions of programs such as AVG and Zone Alarm do distinct jobs and there is no risk of conflicts. Upgrading to one or more paid versions would not only be a waste of money, it could lead to possible system conflicts because the functions of these 'advanced' versions often overlap with each other.
So, if you simply download AVG Free 7.5 (and Zone Alarm Free if you've not already got a decent firewall), there is nothing else you need to do.
Chris