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When my users says that they need more disk because their system is slow, I use the house analogy saying that if their closets are full, that does not prevent them to walk around in the house at full speed. The disk is like a closet, you can add more storage, it has an effect if clothes are disordered, in that point I can use that analogy to explain fragmentation if needed.
About explaining antivirus, my successfull analogy is that an AV is like a guard that is at the door and check everyone entering against a list of bad guys. And to update the AV is when you give a new list to the guard. And scanning for virus is when you send the guard inside to check for hidden bad guys under the rugs, behind the beds, and so forth.