avast is an antimalware product that uses virus signatures, behavior analysis, generic and algorithm detection, code emulation and so on to prevent malware in your computer. Each Shield has a particular task in the protection scheme and it's described in the help files.
avast has a boot time scanner (exclusive), a very good update scheme, a strong community, etc.
Let the default settings that your computer is protected.
Get used to avast first, before tweaking the settings.
