I haven't seen any recent statistics, but at one point, 74% of rootkit infections were on pirated copies of Windows XP [0]
[0] http://www.zdnet.com/article/study-rootkits-target-pirated-c...
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...