You are having computer problems. You have Microsoft software. You receive a call from someone telling you that your computer has problems needing to be fixed: it has a virus; it's slow, or it's sending out error messages. How wonderful! Someone at Microsoft knew of your troubles. What a great coincidence! Help just when you needed it.
But Microsoft does not call people. These callers hope to find those people who are actually having trouble. If you talk with them, they will ask you to turn over to them control of your computer. They will hook you up to their computers through remote desktop software so they can "fix it". Many tech helpers do just that, ask for your permission to take over the controls. If you allow them to remotely control your computer, these scammers may be able to find your passwords and accounts and raid them. (See this online discussion group.)
The con artists are using probability and probability alone. They guess that one of their many calls will find someone with computer problems (always happening to someone), who does not know that Microsoft never calls. Nothing personal. Just numbers. If you bite on this coincidence, it will become very personal.
How Madoff made off with other people's money and financial newsletter writers can trick you into subscribing. See Connecting with Coincidence p. 164-6.