There were presumably computers there before on the other rovers. They either had Windows or a third OS or no OS according to the title.
VxWorks, specifically.
Some cyclic executives and such, too.
Given the relatively slow CPUs and limited memory on those older rovers I would expect an embedded OS before Windows. Think QNX or VxWorks, not Windows NT.
Perhaps when Russian or Chinese probes successfully land. They are both notorious for "borrowing" US software.