Quote
Internet Explorer users who visited a Web page containing this ad and whose IE was not equipped with the WMF patch would not get that warning. Rather, their machines would silently download a Trojan horse program that installs junk software in the PurityScan/ClickSpring family of adware. This stuff bombards the user with pop-up ads and tracks their Web usage. Only a little more than half of the anti-virus programs used at anti-virus testing service AV-Test.org flagged the various programs that the Trojan tried to download as malicious or suspicious.