Microsoft would, in theory, be able to build a Windows XP variation of DirectX 10. Reports by other tech websites say that this would be very costly in development expenses though. Even though it might just be that Microsoft would want to get of the complete virtualised support of graphical pc components train. Reason why? Answer: nVidia..
nVidia got quite a few problems developing good drivers for Windows Vista that should make use of virtualised memory. In fact it seems that they have had so many problems with it that they still havent managed to put a set of decent drivers on the market with decent results. Unlike AMDs ATI does not seem to have such problems at all, it still seems that Microsoft wants to help out nVidia.
By making virtualised memory optional it would help out nVidia overcome their drivers problem, but such a drastic change would also have another effect. This is that currently the virtualised memory component is the biggest problem to make a good DX-10 port to Windows XP, but if this component becomes optional The Inquirer might just be right about a DX-10 port becoming relatively simple. The development costs would also be a lot lower.
The big question however is if Microsoft would want to do this. So far Vista hasnt had much support from gamers. Where the problem of useless nVidia drivers for Vista was one of the reasons and if it ever comes to DX-10 port to XP it would just give more reasons to not make the switch to Vista for now.