The PC maker has been emailing customers advertising the discount, saying that Windows 7 is "back by popular demand."
Specifically, it's Windows 7 PCs for $150 (Â£91, AU$170) less than its Windows 8 machines.
In addition HP's retail website currently features Windows 7 PCs much more prominently than Windows 8 PCs, in the laptop section and especially in desktops, where it's not displaying any Windows 8 machines at all.
Time for a change
It's unclear why HP is emphasizing Windows 7 over Windows 8 so adamantly.
But it may be right about the "popular demand" thing; Windows 8 launched to severe criticism from all sectors of the tech industry, from consumers to reviewers to developers.
Even Microsoft's own employees have admitted that Windows 8 is "the new Vista," referring to the oft-maligned Windows 7 predecessor Windows Vista. And a Samsung executive said much the same thing early in 2013.
While in TechRadar's definitive Windows 7 review we called it "the best Windows operating system ever."
Windows 9 is launching in 2015, and many Windows users are hoping for some drastic changes to Microsoft's OS.