Windows is not an Open Source OS, it's 100% proprietary. It's an Open Platform, which is what I think you mean. The difference is you can write any program you want to run on Windows, and distribute anyway you wamt, but you can't alter the Windows source code, which you can with an true open source OS.
I think for consumer focused devices like phones and tablets the closed model of Apple and Microsoft is better than the open model of Android. As a developer I think submitting your applications to Microsoft and Apple for review forces us to write more standard compliant and efficient code.