The old one has been around since the days of the Berlin Wall and Matthias Rust, and although we are Cold War buffs it was long overdue for a redesign.
The new logo just looks sleeker and it should match the look and feel of new Microsoft operating systems much better. Aside from a new square symbol with no bells and whistles, Microsoft also changed the font and dropped the italic look.
The logo does not feature any rounded corners, so Apple could have a harder time filing frivolous lawsuits. As we all know Apple somehow managed to patent a shape invented by butt naked men with chisels in the stone age, whereas Microsoft was apparently more influenced by monkeys breaking bones in front of an Arthur C. Clarke monolith. [I had no idea Ballmer starred in 2001. Ed]
You can check out Microsoft's official logo video below and find more info here.