Recently, I read a news about that there is PDF flaw. I am not sure how is it happened. But you can read a news about this PDF and trojan problem:
Beware of the PDF file when you receive a spam mail. The PDF file will disable the Windows Firewall, and download the trojan file.
I also read an article talk about that some people like to run the Windows with reduced rights (non-administrator rights). Because this can disallow the modification of you Windows system, such as installing unwanted programs.