"Scammers are at it again – taking advantage of Google sponsored ads for acquiring traffic in order to redirect it to malware-infected copies of legitimate software. win.rar GmbH is warning users of an ongoing fraudulent AdWords campaign pushing a malware-infected copy of WinRAR
...."

http://blogs.zdnet.com/security/?p=2405

and

'Exposing fraudulent Google AdWords'
http://ddanchev.blogspot.com/2009/01/exposing-fraudulent-google-adwords.html