Why Silverlight+XNA? What inherent advantages does it have?
Anyway, no, your strategy won't work: Windows 7 requires x86 hardware and there is no low-power x86 hardware out there, you can't build an x86 tablet with reasonable 3D performance that has more than a few hours' battery life and with a diminutive form-factor. The iPad can play 10 hours of video non-stop, you can't do that with x86, and you know the next iPad will be even thinner.
What he said ^^.
Once you've decided to run Windows on an ultramobile device, you've already failed.