There are rogue programs, computer viruses, spyware and scareware, which "scare" the computer user into thinking their computer is infected and tricks them into buying a useless program.
However, those pale in comparison to the next big threat: Ransomware.
Symantec details the threat in a document that it's made publicly available.
Briefly, a criminal gang seizes control of your computer, claiming to be law enforcement (or in at least one instance, Microsoft) and demands payment within seven days to unlock it
The nefarious programs usually are picked up while visiting porn sites, which makes the victims all the more likely to pay whatever the criminals demand. Especially, if it's a work computer.
Symantec says it believes that gangs are extorting at least $5 million a year from their victims.
Read more about ransomware in this Symantec PDF.