Yes, but the caveat to the caveat is that Windows 10 will both be free and is intentionally being made to run on very dated hardware. As far as has been released that I can see, 10 will run on anything that will run Vista (which was released eight years ago and ran on Pentium IIIs and Celerons.) While this won't necessarily make the army of XP machines that still live out there go away, Microsoft is pushing really hard to get everyone up to date on the OS.

Source: I attended the Microsoft Spartan preview event last week.