Let's "ramble" this one out step by step.

500 raindrops per second means that you have 500 times 0.00005 kg striking the windshield per second.

So in one second 0.0250 kg of mass at 16 m/s strikes the windshield. This creates a change in momentum of 0.4 kg m /s in that one second. This is, in turn, equal to the impulse exerted on the windshield in one second: 0.4 Ns. Thus the force on the windshield is 0.4 N s / 1 s = 0.4 N.

-Dan