This is an FAQ on the forums. And unfortunately but unsurprisingly, the answer is no, you cannot build a Metro app without Windows 8. If you are running Windows 7 and don’t want to risk screwing up your OS, you could install Windows 8 on a VM or dual-boot off a VHD/2nd partition. Those options are reasonably performant, and more so if you have an SSD.
Building Metro apps without Windows 8
