In two decades of using PCs, I learned one thing—never to rely on default options. If there's an option to customize something, use it; not necessarily for changing, just to see what it offers! Using express option when installing software and getting random bullshit installed along does this to a human.
Same applies to security software. God knows what they mean and what settings they use for full scan. I'm always using custom scan option; that way I'm absolutely sure entire PC will be scanned.