I think the expectation is by the lousy spammers is if they send enough SPAM, eventually someone is going to click their link and inadvertently download and install their malware. I work on computers as a side; people have gotten better but you'd be surprised how many unprotected AND under-protected computers are out there. The link below explains what the SPAMMERS are trying to accomplish better than I can so you can read it if you'd like.

http://en.wikipedia.org/wiki/Zombie_computer

Regards,
Bill