Firstly, always try to use the latest OS with latest updates.
Although I usually go with a mix of paid software and free ones.
For Free software
I suggest using a good anti virus like Avira / Comodo instead.
Comodo comes with an inbuilt firewall and is comparatively lighter on memory than Avast.
Also, I suggest turning on BitLocker encryption for additional benefits.
And using Keyscrambler for safety when typing passwords in browsers like Firefox/Chrome/IE
For Paid software
I suggest going for Emsisoft / GData / Bitdefender / Malwarebytes. You can try using them through giveaways as well.
I also suggest investing in Keyscrambler & cFosSpeed as I have personally very good experiences with these paid ones.
Addons
Recommend using AdBlock Plus and NoScript in both Firefox and Chrome.
Recommend removing greasemonkey as it potentially poses a security threat through scripting support.