History, it seems, repeats itself with the OLPC project. Shortly after founder Nicholas Negroponte announced a dual-boot Linux/Windows XP version was being developed with Microsoft, including possible support from the Bill & Melinda Gates Foundation, Microsoft have themselves debunked the story and denied their involvement:
"While we have investigated the possibility in the past, Microsoft is not developing dual-boot Windows XP support for One Laptop Per Child’s XO laptop. As we announced in December, Microsoft plans to publish formal design guidelines early this year that will assist flash-based device manufacturers in designing machines that enable a high-quality Windows experience. Our current goal remains to provide a high-quality Windows experience on the XO device.
In addition, there will be limited field trials in January 2008 of Windows XP for One Laptop Per Child's XO laptop. Microsoft recommends contacting the company directly for any further updates" Microsoft statement
It seems to be the case that while Microsoft is keen to ensure Windows can run on small, low-power computers - which is of course in its best interest in terms of OS sales - a willingness to experiment with whatever machine you've developed doesn't necessarily mean it'll be adding its name to said-device's list of proponents.
OLPC are yet to comment on the matter.