It is our opinion that computer should be upgraded as you go along. A good computer should last 8 or 9 years from the date of purchase.
Antivirus software is a program to help protect a system from known virus but will always be only as good as the latest update. When a new virus is created most antivirus companies don't know about the virus right away. Once they become aware of the virus they can update the definitions so they can protect the computers they cover. Free antivirus usually only scans a limited amount of files on the computer and will use older definitions. This is one of the reasons we promote our managed Antivirus. Not only will it let you know if you system has a virus but it will let us know as well.
Tables are able to do several things and for some it can be enough. I find it to be far to limited as a computer replacement at this time. Most tables are designed to be usable only for a few years and then need to be replaced. Limited storage ability will mean that data will have to be stored in the Cloud. Most tables can not be upgraded. They are what they are.