Dear Stangnet.com.....

Most "infected" ads or files are found on, shall we say...

sites with "questionable" content? :D

Not that I would know these things...

I've heard, though...

From my friend's neighbor's buddy...

Normally, I agree. However, it appears that Yahoo and to a smaller extent Google have been unknowingly delivering malware ads. Check this article out:

Malware delivered by Yahoo, Fox, Google ads | Tech News on ZDNet

Scary stuff. You can protect yourself yet still get nailed.