Last year, I wrote about a virus that is responsible for the top six Explorer crashes, by a wide margin.
I learned later how the authors of this XYZ Virus operate, and it happens to answer a question posted by commenter SteveL as to why these virus writers are so incompetent that they crash so much.
First, the virus authors infect your computer and crash your system every so often on purpose. Meanwhile, they also set up a legitimate-looking Web site which sells anti-virus software that claims to remove this virus. You send them your money, they send you the software.
The kicker is that the removal software doesn't work. Your computer is still infected with the XYZ virus. But they don't care. They already got your money.