system
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Yes, gone are the days when a simple AV such as MSE was sufficient sitting in your tray and updating once a day (if you were lucky). The hackers/malware kids are getting more sophisticated and so the AV studios have more and more bases to cover.
Plus a custom install is something everyone should do, not just for Avast but for every program they download (if it has one). Otherwise inadvertent downloading of unwanted programs that are often bundled in downloaders can creep through un-noticed.