Wow, so many sales pitches against Avast. Did the OP ask "What should I use instead of Avast?"
Use whichever AntiVirus you prefer, they are all just as good (bad) as the next as far as degrading system performance and catching infections. I have used Avast as well as AVG, MSE and Kaspersky on multiple machines (not all on the same machine) for years and never had issues.
I do recommend using Malwarebytes which you listed specifically as it is a great program, but I recommend buying it so its active scan can actually protect your computer, which anti virus "active scanning" (aka non stop HDD dumps) generally don't prevent, and to support the awesome developers of such a great application.
Spybot S&D has a great blacklist which will keep your computer from getting many nasties, but there are other free whitelisting/blacklisting applications around.
As long as you have active spyware protection and any antivirus software (even Microsoft security essentials is good and comes with Win8) you should be safe from the general infections.
I always add a white/blacklist also because I like to browse my pron without having popups / system crashes 2 minutes into the videos