No matter how many times I repeat this, I still find myself consulting with people who fell for scams.

1. If you get a phone call from "Microsoft," "Apple," or any other company that is calling you out of the blue, it is a scam.  Theses companies will never call you, and are hard to get hold of if you did want to contact them.
2. If your screen is taken over by a warning of any kind that tells you to call a certain number, it is a scam.  No legitimate company would lock you out of your computer.
3. If you get an email that your password is expiring or your mail is not being delivered or any other request that requires you clicking a link in the email, it is a scam.  If you're not sure, go directly to the website of the company in question, do not use the links in the email.