I think that this may actually be attributed to the lack of working Windows Update on a lot of pirated systems.
According to Microsoft[1], 32% of pirated Windows 7 copies and activation cracks resulted in some sort of malware infection
Speaking anecdotally, most of the friends and family whose computers I've had to clean up have been running some sort of cracked/pirated software that they'd downloaded or had been given to them by a friend
[0] http://voices.washingtonpost.com/securityfix/2009/05/pirated...
[1] Admittedly, Microsoft has somewhat of a bias, but the number sounds reasonable to me: http://archive.news.softpedia.com/news/32-of-Pirated-Windows...