It happened to me a few weeks ago. My laptop locked up and I got a message, supposedly from the FBI cybercrimes division, saying that there was a criminal investigation being launched against me for one or multiple infringements. And being the tech savvy guy that I am, I thought it was real, and started freaking out. The letterhead seemed authentic, and it had my name, IP address, and service provider. Fortunately, after the intitial shock wore off, I was able to think more clearly, and realize it was a scam.
For starters, the fact that it was asking for 300 dollars, that could only be paid with Moneypak, should've been a red flag. And then there's the fact that the FBI would never employ such a tactic to begin with.
So I was without my laptop for a couple of weeks because I wasn't aware of how easy it actually was to remove the virus. I thought I was going to have to take it in to a professional, and wasn't sure I wanted to bother, being that it was an old computer, and I've been wanting to update. But I got bored last night and was able to remove it on my own with little difficulty.