On Windows, how do you get time in nanoseconds in C?

Short answer --- you can't and for good reason --- it won't be accurate. The best that is available is 0.1 milliseconds using the QueryUnbiasedInterruptTime API and even that value is suspect.

Extreme time accuracy is simply not attainable in a multitasking, interrupt driven environment like Windows.

And this is probably not the appropriate forum for such technical questions.