Once Microsoft's closest partner, HP is doing everything it can to distance itself from Redmond these days. Case in point: A new ad campaign for a range of PCs that eschews Windows 8.x for an older offering, Windows 7. The message? "Back by popular demand."

Some will accurately point out that big, corporate-focused PC makers like HP have long offered previous Windows versions to their business customers, and of course Microsoft itself lets volume licensing customers "downgrade" to previous versions too. But there's never been something quite this public, let alone an ad campaign that is clearly targeted at end users, and not businesses.

According to promotional materials linked to the ad, which was sent to customers via email, the PCs are brand new—"our latest HP notebook and desktop PCs"—but the OS, Windows 7, is not, having been succeeded by both Windows 8 and Windows 8.1 since its launch in late 2009. Looking over the PCs HP is offering, they appear to be traditional PCs in the sense that they don't offer multi-touch capabilities. And three of the five PCs are desktop computers, not portable computers.

That this strange offer comes after a year in which HP embraced both Android and ChromeOS is, perhaps, not a coincidence. Indeed, all of these offerings—the Android devices, the Chromebooks, and now these Windows 7-based PCs all come in the wake of both Windows 8 and Microsoft's Surface PC lineup. As I described previously in Post-PC: HP Declares War on Microsoft, HP CEO Meg Whitman seems hell-bent in destroying whatever cozy relationship the two firms once had.

"Long-term HP partners like Intel and Microsoft are increasingly becoming outright competitors," Whitman said last year, suggesting that the rift is Microsoft's fault. "The number of personal computing devices worldwide is exploding."

Also not a coincidence: HP is no longer the biggest PC maker in the world. That title now belongs to ThinkPad-maker Lenovo which, it should be noted, also sells Android-based tablets and Chromebooks. The difference is that Lenovo and Microsoft were never really exclusive. HP and Microsoft very much had a special relationship.

Not anymore. And now HP is actively selling Windows 7-based PCs, and helping to undercut Microsoft's latest OS efforts on variety of fronts. With friends like these...