Wikipedia in russian says Windows 95 is result of unification of products DOS and Windows that earlier were distributed separately. However I found something that says it differently.
"Windows 9x/ME: the 16 bit subsystem was salvaged from DOS and Win 3.1 and therefore was written in assembly language; the rest was a combination of assembler and C. Win 9x was designed to work well on suboptimal, older hardware (486s and such), therefore needed the speed advantage of assembly language. Remember that Win 95 was originally packaged as an upgrade from Windows 3.1.