So How Did I Get Infected Anyway?
For quite a few people it's by installing programs like Messenger Plus,
whose ads for malware don't identify the malware as such and try to convince
you that you owe it to the author. See also: http://www.wilderssecurity.com/showthread.php?t=27971
Don't ever do a "default" install of anything. Always choose Custom and see
what else is being carried along. Don't install any extras you're not sure
of.