It's no secret that Google purges dangerous pages from its index of searchable sites, but apparently that hasn't stopped the company from selling text ads to a known malware purveyor and placing them on a number of websites affiliated to the AdWords service.
The link was listed as "Antivirus XP 2008," which is the name of one of the most widespread and obnoxious bits of malware floating around the internet. The bogus antivirus program hides itself in your system and pops up at intervals with an alarming message that claims your computer is infected with all sorts of bad things. It then recommends that you buy the pro version of the program to clean up the mess, but of course this pro version also does nothing except rip you off.
Google reportedly shut down the ad when notified but probably not before it fooled many non-experienced users. The oversight is particularly surprising, considering the search giant could simply cross-reference the list of purged sites with the list of advertisers to avoid this kind of mishaps.