Microsoft could get a pretty good deal on the vacant Circuity City buildings slated for public auction.
In Salem, NH, they would be better advised to move into the still-vacant CompUSA store. It's right across a parking lot from Best Buy, which is still alive and drawing consumer electronics buyers. CC is by itself.
Another option would be Rockingham Mall. The former Jordan Marsh / Filene's space is still empty. A bit huge for MS, but it's right around the corner from their nemesis.