It will come right off with a razor blade. Go to the dollar store and get a razor blade scraper. I am a painter and it works like a charm.
What FC1 said.
You might try experimenting in a small corner with non-acetone fingernail polish remover on a cotton swab or ball to see if that makes the job easier. Be sure to have plenty of ventilation as you work though because even the non-acetone remover has fumes that can leave you light headed.
You can use a metal paint scraper or putty knife. It'll take work, and you'll have to be careful not to scratch the glass, but it should come off. There are also really nasty chemicals that you can coat the paint with first to soften it up before going after it with the scraper, but if you go that route, please be very careful about ventilation; they're extremely toxic. You should be able to get that stuff anyplace you buy paint. Since you won't need much if you're only doing one window, it shouldn't end up costing a lot.
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