These are usually caused by visiting 'suspect' web sites.
The way to avoid them is, don't do that.
Here is Norton Securities take on it and how they tell to avoid it;
(By the way, do you know the web site link I gave is safe?)
How can this thing be stopped, Before it get into your pc? I am using Malwarebytes-Pro, MSN Security Elements, and still got hit twice, encrypting my text and jpg files.