Seems there is a trojan out there that can replace Google AdWords with other ads, according to BitDefender. The bug is loaded when a person visits an infected website and the malware attaches itself to a user's computer.
Then when they surf a publishing site that displays AdWords the ads are replaced by similiar looking ones from other advertisers.
"The threat, which is identified by BitDefender as Trojan.Qhost.WU, modifies the infected computers' Hosts file (a local storage for domain name / IP address mappings, which is consulted before domain name servers and is considered authoritative).
The modified file contains a line redirecting the host "page2.googlesyndication.com" which should point to an IP of the form 6x.xxx.xxx.xxx to a different address, of the form 9x.xxx.xxx.xxx, so that the infected machines' browsers read ads from server at the replacement address rather than from Google," BitDefender noted.