No doubt, many open source projects compile with many warnings. Not that this is an excuse, but open source projects do have a bit of an uphill battle on this front, as there are many compilers out there, and unless they have a build farm to check against them all with, they're not likely to fix everything. [Again, not to make an excuse or minimize your statement -- most warnings I've seen in open source code are easy to fix and not compiler dependent.]
I would imagine its fair to guess that Microsoft builds their OS using the same compiler that Visual Studio uses. Ever work in that environment? Perhaps there are ways to elicit better warning output, but the default (at least when using the UI) is that many warnings that GCC (and other similar class compilers) output, Visual Studio lets go without a peep. That can make code compile warning-free, but in no way is the number of warnings a promise of comparability to quality of what gets produced, eh?