If you ran a Google search for “download windows 7” early Monday, you got about 137 million results — plus something you probably weren’t looking for. There, at or near the very top, was a sponsored link suggesting “There’s never been a better time to switch to a Mac.”

A four-line ad was spotted over the weekend by the Next Web‘s Alex Wilhelm, who suspected that Apple (AAPL) was paying exorbitant prices for the space.

In fact, this form of guerrilla marketing is surprisingly inexpensive. The cost per click for either “windows 7 download” or “download windows 7,” according to Google’s (GOOG) Keyword Tool, is 5 cents.

[UPDATE: The Apple-sponsored link disappeared mid-morning Monday, but it was back early Tuesday.]

See also:

• Apple’s 2009 ad budget: Half a billion

[Follow Philip Elmer-DeWitt on Twitter @philiped]