Microsoft published a research paper outlining a P2P file-sharing system similar to BitTorrent.
The Microsoft P2P software project has the code-name Avalanche. Microsoft's approach tries to solve performance issues with what they call network coding. There is no word on when Microsoft is going to release their P2P file sharing software.
More details on Forbes.
BitTorrent inventor Bram Cohen takes apart Microsoft's Avalanche, calls it garbage. Read his blog for more.