DX10 will only work on Treacherous Computing (a.k.a. "Trusted Computing" by the "industry") enabled hardware. That's why Vista needs so high specs. Because new hardware are TC enabled. If Vista allowed old hardware, it would not work with TC and thus the users can't be controlled properly with TC, DRM and so on.
If you could install DX10 on Windows XP it would totally ruin MS goal to put everyone under control and surveillance on Vista. Because XP is not TC enabled, and the hardware it runs on is not either. Be prepared that IF they ever release a patch for Windows XP, expect that they claim you need to upgrade the hardware as well, or it won't work as you want. The truth is that it won't ever work as you want whatever you do.
TC is a scheme to encrypt all internal and possibly some external communication on your computer hardware. In this way you can't do what you want to do. There will be a small program in the hardware that monitors what you do to make sure you don't do anything "bad" with your own computer (such as copying a movie, downloading music, playing a pirated game and so on... maybe also make sure you're not getting services from competitors). Vista is capable of respecting this little Big Brother (the hardware). You can't change Vista, because it's proprietary, you can't change Big Brother because he's encrypted with a secret 2048-bit key and won't work unless he's encrypted (the only other option is to not use the computer at all).
Initially TC was supposed to allow you to decide what the hardware ("big brother") should allow or not. The industry quickly made an incompatible design that NOT lets you decide. Therefore there can be some TC that is usable, just as there are DRM that is usable (i.e. for
personal stuff)... It's however unacceptable that TC is used
against you to make sure you don't infringe on what the industry call their "intellectual property".
Read more about Treacherous Computing here:
http://www.lafkon.net/tc/http://en.wikipedia.org/wiki/Treacherous_Computing