Technically speaking, no. The mantle, which is solid, comprises about 2/3 of the Earth’s mass. However at a planetary scale solids are not rigid enough to maintain their shape, so the Earth is closer to a liquid held together by gravity than to a rigid solid object. See this simulation for an interesting demonstration of its properties: https://youtu.be/kRlhlCWplqk
I think a two disk dual boot is safe. I’ve had that setup for a while and Windows hasn’t broken anything yet (though I only use it maybe once a month).