Do a search on "download Windows 7" or "Windows 7 download" and you'll see the Apple ad on the right-hand side of the page, under the "Sponsored Links" section. Some times when I do the search, the ad instead appears on top of the main search results.
The ad reads:
Upgrading to Windows 7?You can see the ad, below.
There's never been a better time
to switch to a Mac. Find out why.
But do a search for "buy Windows 7" or "get Windows 7" or "purchase Windows 7" or "Windows 7" and no ads from Apple for Macs appear.
Is Apple doing this as a way to target Windows 7 pirates? After all, if someone is looking to find and pay for a copy of Windows 7, they're likely to type in the word "buy" or "purchase" rather than "download". If, on the other hand, they're looking to pirate a copy, they're more likely to want to download it, so will more likely type in the word "download."
Why would Apple target Windows piraters? Frankly, I don't have a clue. And I'm not completely convinced they're targeting pirates, either. So if you have an answer to the mystery, or a theory about it, let me know below.