Hewlett-Packard (HPQ) is offering customers an instant rebate of up to $150 for ordering desktops and laptops equipped with Windows 7. That’s right, go to HP’s website and see for yourself. HP’s gone back to the future, promoting Windows 7, not Windows 8.x, in a campaign it’s calling “Back by Popular Demand.”

Visit HP’s desktop page and right away you see three models prominently featured as Windows 7 offerings, offering savings ranging from $80 to $150. While the desktop page provides a Windows 8.1 features tab, it’s positioned well below the PCs. The same holds true on the laptops page, with three Envy entries offering savings from $100 to $150 presented as Windows 7 units. HP’s all-in-one PCs are shown as Windows 8 models.

Is HP thumbing its nose at Microsoft (MSFT) over Windows 8, or is the systems vendor simply looking to spark PC sales? Once an unquestioning Microsoft loyalist, HP is the first PC manufacturer to overtly promote Windows 7 over Windows 8. Will others follow?

In the last year, HP’s PC shipments slid 8.5 percent as the vendor lost its overall segment leadership position to Lenovo, which, along with Dell, grew shipments by 9 percent and 6 percent, respectively, according to IDC. It’s possible other PC makers such as Acer and ASUS, which suffered worse losses, could take HP’s lead in hyping Windows 7-based PCs over Windows 8 systems.

There’s some history here of PC makers showing a preference for an older Windows over a current version. Microsoft Vista was a disaster, never living up to the hype and confusing users with an overabundance of versions. Befuddled OEMs eventually gave up and went back to Windows XP.

With Windows 8.x, try as it might, Microsoft has failed to convince a good swath of users of its benefits, leaving many still perplexed over its confluence of desktop and touch-tablet computing styles.

As for HP, last year the vendor showed an inclination to carve its own multi-OS path, coming out with Google (GOOG) Chromebooks only days before Microsoft shipped Windows 8.1 and offering an Android-based tablet. And, two recent reports say HP is set to unwrap a low-cost, Android-based smartphone aimed at emerging markets.

We’ll have to see how long HP keeps up its back-to-the-future thing.