My favorite story about this is I had a friend who was smart but quick to jump to conclusions and not technical.

He had a roommate who sometimes used his computer. He comes home one day to a message on the screen, full screen saying "This computer is locked by the NSA. We have detected illegal activity including child pornogry(yes spelt that way). You may send $300 in Walgreens gift card codes to (some yahoo email address I forget) or you will be arrested. This computer can not be unlocked."

He panics thinking his roommate did something, shut down the computer, removed the hard drive and memory.

Two years later I hear about this and tell him it's fake and let me take a look at it. Turns out the computer was not" locked". I got into the system by booting into safe mode. There was a program added to the start up folder of the start menu(this was in windows xp). That program literally just launched a window with that message, set to full screen with no close button. Out of curiosity I rebooted the system message popped up and I just hit alt-f4 and had full control of the system. These scans are designed for people to panic and not think critically.