Apparently people are uploading malware to users computers in order to modify ads displayed on websites they visit with their own ad.
"Techshout.com reports that a new, deceptive Trojan Horse program has surfaced. The program is engineered to produce fake Google ads that are formatted to look like legitimate ones. The ads are incorporated in Google AdSense, the program that lets website owners display ads from Google's list of advertisers. The Trojan Horse apparently downloads itself onto an unsuspecting computer through a web page and then replaces the original ads with its own set of malicious ads." - TechShout