So I was thinking about viruses recently, and wondering how exactly antiviruses keep up? Considering anybody who'd been coding for a few weeks could hack together something do do nasty, nasty things to somebody's PC, the quantity alone would make a simple list of hashes prohibitive, so how do antiviruses do it? Do they monitor process activity and have a 3 strikes rule for doing virus-like things? And if so, what's stopping it from triggering on perfectly harmless things (like me moving files around in \system32)?

I did a bit of googling, but the regular places didn't particularly help, and I couldn't find a dupe here, so I thought it'd be good to ask :)