Hi there Bob!
There isn't always a "cut and dried" answer to how people became infected. Was it through an email attachment they shouldn't have clicked and opened? Or perhaps a bad link via Instant Messenger? Many p*rn sites will load spyware and malware, and most P2P type programs do, as well.
Not having a firewall is another good way to get infected! Let's not forget people who never read EULA's when they agree to something...and end up getting Malware "extras" installed with their programs.
There are SO many different ways, it truly would be impossible for us to tell each person how they got infected.
Rather...we all use "prevention" speeches when we finish a post, to help people STAY PROTECTED and learn how to not get this stuff on their system.
Check this out..as well!
see TonyKlein's good adviceSo how did I get infected in the first place?
and AntiSpyware Net's spyware article: Spyware, Adware, Malware: What it is, how it got on my computer, how to get rid of it, and how to prevent it.