Actually, if they do this store right, it will most likely increase their market share in all the fields they sell in.
MS already throws enough money at their core markets. No one still comes close to the dominance of Windows and Office. Windows Vista was a hick up, but it wasn't for lack of money, it was too much corporate oversight, and them not working so good together. When they went into making Windows 7, they changed how they usually make Windows. Because of that, they have a awesome OS on their hands.
And MS is actually happy playing no. 2 in sections that are not Windows and Office. They are really planning for the long time future.
MS has Bing, now coupled with Yahoo. Their market share in the search business now jumped from the 8% (it used to be 3% before Bing) to 25% of the search business. They are going to let Yahoo do what it does best, and sell adds etc.
Xbox 360 has been a loss leader for YEARS and now they are finally starting to see some minimal return on it. And coupled with all the new services that keep coming out with the 360, it could easily be dominant with the next gen console, or hell with the 360 still. Because Sony messed up up it's release of PS3, all the cards started aligning for MS and Nintendo.
The Zune is actually a great device. It is better than the non-ipod Touch devices, and with the Zune HD, it will simply be better than all iPod's. It is actually getting lots of hype and praise from usually hardened MS bashers. A lot of people are excited for the devices release. A lot of people wish it would be made into a phone, and it might be. Who knows. And now the Zune will tie into the 360.
360 and Zune are now one team (coupled with another team or something) so better intergration of products is happening.