Microsoft are NOT forcing you to upgrade. The reason it is Vista only is because it requires DirectX10. Which is Vista only as it required modification of code at a level not attainable without designing a whole new OS.
As has been stated above (and is pretty common sense), the game has the ability to run in DX9. It would be madness to make it DX10 only as i bet less than 10% of pc gamers actually own a dx10 card at the moment. I imagine it wouldnt take much work to make a copy for other operating systems that only contains the DX9 part, so yes MS are forcing us to buy vista as they did with halo2 and will do with crysis.
Its not a bad operating system, i've got a copy and have no complaints so no worries.