Do you have a bunch of applications installed? IIUC, Windows keeps different versions of each of its DLLs around for whatever apps requires them. I would expect that installing applications might embiggen your Windows directly a lot.
I assumed it WinSxS related as the other reply mentions, and upon investigation found that it is 9GB alone! I don't run Windows outside of work so I've just never really bothered looking into it, but if it continues to balloon I guess I'll have to.