Follow the Bouncing Malware -- ISC Handler's Diary
Tom Liston wrote up an excellent (and scary!) analysis of what happens to an unpatched machine when it goes to a less than reputable site. The full details, part 1, part 2, part 3 and part 4 are well worth a read. You’ll be stunned at how much shite comes down from a single executable that the user never even gets a choice whether to run.