They hit a wall, they have Windows, Office, and Xbox. Nothing else they were working on was big enough. Also, the only thing they worked on during the Internet boom was IE, and it just does not make them money like Google can by integrating adds with Search.
They still make a ton of money, but its basically around the same each quarter, so its more of a dividend stock. You get 2.5% dividend on your money, the growth days of MSFT are in the past.