http://blogs.mercurynews.com/aei/2006/05/an_investigativ.html
I've got a copy of an email about a project called Helium at Microsoft. The email came out a couple of years ago and it apparently describes an attempt to put Windows on the Xbox 360.
I hear from my sources that this was the third SKU, the version of the Xbox 360 that ran Windows. It is the ultimate Trojan Horse in that respect.
They reportedly canceled Helium. I hear that it was an idea they tried out to please Bill Gates' curiosity, but it never panned out. But I don't know that for sure.