Report of Virus by User

Hi, one of the users of a WordPress discuss forum I moderate told me that when he got to the blog, a virus was downloaded. He has a windows machine, and it was one of those pop-up ads that says the computer is compromised but you can purchase the fix, then it proceeds to try to download malicious text. I could not reproduce the problem. Does this type of thing occur frequently? He is pretty convinced it came from the WordPress forum, but I suppose it could have just coincidentally sprung to life as he was trying to access the form.

Thanks.