MPC-HC only supports DXVA bitstream decode; it doesn't support partial GPU acceleration (and even if there was such a support, it wouldn't decrease CPU load dramatically).
GPU bitstream decoding is only available when using DXVA 2.0.
DXVA 2.0 is a feature of Vista and Windows 7. Microsoft didn't backport DXVA 2.0 to XP, and it would be impossible to backport it to XP without all the features of the new Windows Core.
1) Intel has nothing to do with your issue;
2) There is no point in complaining that MS didn't backport DXVA 2.0 to XP; it is like complaining that MS didn't backport Aero Glass to MS-DOS.
DXVA1.0 is enough to play 1080p using GPU and it is availiable under Windows XP and one can use it in MPC-HC.
DXVA is quite picky, so if you want to use it you have to respect those rules :
Windows XP users, select Overlay Mixer, VMR7, VMR9 or VMR9 renderless
Does Intel HD Graphics support DXVA1.0 or not, that's a question. If not I guess there is a way to emulate DXVA1.0 through DXVA2.0 or something. As i see Intel's driver for Windows XP is simply junk.