The thirty day period you referred to is for a trial period of a paid version/product.
I cannot understand why you wish to stay with an outdated version (v7.0).
v2014 is lighter, quicker and has the latest technologies and provides better protection.
That said…
Download and save http://files.avast.com/iavs9x/avastclear.exe
Download and save http://filehippo.com/download_avast_antivirus/13608/
Run the uninstaller for “each” version of avast you have or ever had.
Reboot and install v7.0
Reboot and register.
I would, however, recommend you download and use v2014 http://files.avast.com/iavs9x/avast_free_antivirus_setup.exe
It’s more stable than you probably think. 