He has a genuine concern that the ad's might be faked due to spyware or otherwise (right?). If your not using an anti-virus, take your computer to the store, give it to someone else, then go home. Kidding. Sort of.
Anyway, try AVG Free edition, works great, also Spybot search and destroy (but make sure you download it from download.com, lots of fakes out there.)
Beyond that, clear cookies and etc. IF that's what your worried about. The more likely solution is that it was just a fluke, some people "game" google by forcing their ads and results up on more popular searches, using bots and what not.
-John
Edit: Well you weren't clear, did you mean "This never happened on OSX or iOS but it happened on my Windows PC?" Or just that it was happening on OSX? If it's happening on OSX it's not super likely it's spyware or a virus, at least not as likely as if it's a Windows PC. If that's the case, they are just gaming google. You may try reporting the link to google and tell them how you stumbled upon it.