Only a day after Microsoft fleshed out Windows Phone 7 series at MIX10, we've already got a full dump of Windows Phone 7 build 18.104.22.16877. The files aren't a leak exactly; they've been extracted from Microsoft's own developer tools, also released yesterday. However, before you get your hopes up, this isn't a shortcut to getting Windows Phone 7 onto the HTC HD2.
For a start it's built for x86 processors - after all, it comes from the emulator not a phone ROM - and lacks the necessary drivers and other elements needed to control phone hardware. Even if we had all that, there's more to the HD2's incompatibility than just the number of physical buttons.