Apparently there is a major security hole for PC users who browse the web.
A few quotes from the article to give you an idea of the threat:
"Internet users visiting some of the most popular sites on the Web may unwittingly be downloading malicious code that compromises their computers and sets up a relay network for a future onslaught of spam, a security services company warned Thursday."
"Without the user's knowledge, the code connects their PC to one of two IP (Internet Protocol) addresses in North America and Russia. From those systems they unknowingly download a piece of malicious code that appears to install a keystroke reader and probably some other malicious code on the computer, Houlahan said. "