Knowing if a phone call or email is a scam
Recently a few customers have had issues with phone and email scams. As a general rule of thumb, Microsoft will never contact you to advise you of problems.
Microsoft does not know your email or phone number, they would also have no knowledge of the status of your computer. Anyone claiming to be from Microsoft and have knowledge of your computer is likely a scam.
The same goes for other IT companies that may email or call and try convince you that something is wrong with your computer - they wouldn't know and is in fact most likely a scam.
If in doubt, speak to a reputable IT company before engaging in any phone calls or emails, especially if they are requesting you to purchase something, or to install anything.