You are browsing the internet when suddenly a screen, such as below appears, showing viruses have been found on your computer.  How do you know this is fake?

You receive a telephone call from someone claiming to be 'Microsoft' or 'Web Security' or such like, saying that your computer has some problem and it is sending out unauthorised emails or data across the Internet.

You receive an email from a courier or money transfer company regarding a failed delivery and they need you to follow the instructions in the email.  Have you ordered anything which is due for delivery?

This is a rarer scam but unusually it is quite clever.......

You sell services such as holidays and are contacted by a 'supposed customer' who wants to make a booking.  The 'customer' books and asks if they can send over extra money so that you can pay for some additional service on their behalf (such as airport car rental). 

There are numerous email scams all of a similar type: Some 'official' has access to a bank account containing a lot of money that they need help transferring from the account.  It is bogus and should be ignored.

Probably the most common scam: You receive an email asking to review your bank details for some reason such as a 'Security review of all accounts'.  IGNORE IT!