It'd have to do machine-code translation, and so would be slow as hell. Try running windows in qemu some time to see what I mean.

To be fair, qemu is appallingly slow. Windows in Microsoft's Virtual PC for Mac (on Power PC macs) was several orders of magnitude moe performant.

Machine-code translation can be done well, but that still doesn't mean trying to pack a desktop OS onto a mobile platform is going to be a good idea.