Excellent question. Here's what I would do.
Treat a desktop like a desktop. Perhaps share elements of the Windows OS between platform (.NET framework, kernel, DirectX, etc) but the UI must be diffirent. In case of a hybrid device; let the user pick the experience he/she wants.
Continue the Home Server concept; partner with a company (D-Link) to create a stand-alone box. Don't screw this up. If you want a media box, do it properly. Get some content; pick a movie studio or two to back you up. Get an American TV channel. GET LIVE SPORTS, especially SOCCER!
Realize that you lost the smartphone market. Work towards creating a presence on the incumbents. Price the competition out of the water. Do a proper Office version.
Lower the OS price. Create three versions tops; Lite, Regular and Corporate.
They want an app store. Okay, that's fine. Create a "certified by Microsoft" program that provides some perks and allow people to buy stuff online. Fully-tested software (a proper QA process), no malware/spyware, backups, more generous licensing,
XBoXOne - give a free online experience. Support the indie community more. Don't release 20 variants of the console.
Forget doing a hardware media player like the zune. Do something that allows you to play music on existing smartphones. An app that allows you to stream the music from your media box (as mentioned above).
Pick another commercial product area. Perhaps education? Perhaps extend messaging.
Look at more reasonable pricing for CALs in the corporate market. Give better volume discounts.