The SDK is a development kit, *NOT* a Win7 system on its own accord. It allows to build the viewer so that it will work on all platforms from Windows XP SP3 onwards. You should be able to install it, even under Windows XP (although I did not try and offer no guarantee whatsoever: the Windows viewers built with VS2010 have all been built by LL and other TPV teams on Win7+ systems).
At worst, if the SDK fails to install under WinXP, you could still install Win7 on a second partition, as a dual-boot (even if only for building the viewer under Win7 for WinXP), or use a VirtualBox (or VMware) machine to run a Win7 virtual machine under WinXP in order to build the viewer.
I'm not the person who defined build requirements and compiled the pre-built libraries. If you are unhappy with this, please complain to LL.
As far as I am concerned, Windows XP is a dead horse (and since I'm using Linux for myself and only publishing Windows builds as a free service for Windows users, I can't afford spending any more time than for building *one* Windows binary for each viewer release).
Note also that LL is not providing updated pre-built libraries for VS2010 any more, since they switched to VS2013 for their viewer, so, at some point, and when some newer/other pre-built libraries will be needed (it's only a matter of time before it happens), I'll too have to drop VS2010 build support for the Cool VL Viewer; I did compile a few libraries for VS2010 myself, but, again, I cannot afford spending my precious and sparse free time maintaining a full set of pre-built libraries for VS2010. Keep in mind that I develop and maintain the Cool VL Viewer *alone* (and yet keep it more up to date and faster tracking LL's viewers new features than any other TPV *team*) !