Uninstall your 30-day trial version and then install the full-download package for AVG Free here: http://www.filehippo.com/download_avg_antivirus/ Best to avoid using c|net for the time being; this link is safe to use.
64-bit version comes to 144 MB in size. 32-bit version is 132 MB in size.
Also suggest running any removal tools for any old, including a/v's that came with your system when new, a/v programs no longer running on your system to prevent left-over drivers from uninstalled a/v programs from causing any potential conflicts with your new AVG Free version: http://singularlabs.com/uninstallers/security-software/
Doing so will ensure your system is clean and ready for your new installation.
I uninstalled MS Security Essentials after reading CNET's deflating review of it and installed AVG's AntiVirus 2014, intending AntiVirus Free 2014. Can I simply uninstall the full-blown version, which requires purchase in 30 days, and install the free version? The CNET review suggested to use this in place of MS S. Essentials.