Windows is 100% not an open platform; you guys are falling for a marketing spiel that only ultimately benefits companies like Microsoft, because of monopolies they already have in the computing space. It's the only reason they push the "open platform" mantra in the first place.
Microsoft own Windows, its source code, Direct X, Word, Office, Excel, and all related middleware and supplementary technologies. These are ALL closed-source properties, and Microsoft is ultimately the main beneficiary to their market proliferation. Your "open platform PC" is only within the range of what Microsoft allows through technologies they release and standards they push for, that they have control over.
Your "open platform", at its root level, is an illusion.