Essentially, he would use a command internal to Windows 2000 / Windows XP to display a message which would warn of system errors, even if no such errors were present. The point was to coerce people into paying for a “fix”:
The bogus warnings directed users to download a free trial version of software that would report "critical errors", even on machines that had no registry problems. The program claimed the errors could be repaired if users spent $30 for a full version of the program.
It's impossible to ask for all computing to become fair and legit, but perhaps at least one scammer has learned his lesson. Then again, he probably will go right back to doing the same thing, in a different way.