If you have issues with hard water spots the best and easiest way to get rid of them is HOT undiluted vinegar. Get it as hot as you can stand and wipe the Windows down with it. Let it sit for a minute or two and then wipe again the spots should wipe right off. No scrubbing required.
This also works on shower doors and faucet buildup. Just wrap a rag around the faucet and pour the hot vinegar over it let it sit for 10-20 min and it turns the white buildup to mush.