I've been using Avast for over a decade without any major problems. There used to be some issues some months ago about it being too eager to quarantine totally legit programs, but that can easily be avoided by putting them on the bypass list. If some program gets quarantined, the bypassing option is given while reverting the said program from the quarantine. For a lightweight version of Avast I recommend the custom install and unchecking every unnecessary module. The FileShield is basically the only one you should leave checked, resulting something like MSE with options to completely shut the program down e.g. during gameplay.