The kernel-mode bits of Windows are mostly written in C. There is a little bit of x86 assembly language for some platform-specific things.
I think it has a fair amount of C, but is mostly in asm.
I don't think there is anything in modern Windows that is "mostly asm".
Windows NT ran on the MIPS and Alpha chips, that got rid of most of the assembly. More recently we have 64-bit Windows running on various 64-bit chips.
Optimizaiton technology is to the point such that both C and C++ can be compiled into very efficient code.