Threat of the year since 2012. Attackers enter your PC through unpatched software and encrypt all your files. Can you afford to pay a $600 ransom to get your data back?
They turn your PC into a remote controlled zombie. Your computing power is collectively sold on the black market to send spams, attack others or store illegal content.
Hackers use phishing techniques and keyloggers to get your most valuable passwords and empty your bank- or PayPal-accounts invisibly within seconds.
Potentially Unwanted Programs that mess up your computer with useless browser toolbars, tons of commercials and other bulk that slows down the PC.