The four-color design of Microsoft's Windows logo started with Windows 3.1 in the early '90s and remained in a boxy design with black borders until 2001. That year, Windows XP launched; the borders were dropped and the window became a waving flag, symbolizing exploration and discovery.
The most recent incarnation for Vista and Windows 7 has a shinier look, as if encased in pristine glass. (Must have been pretty strong glass to survive the negative Vista publicity.)
Why red, green, blue and yellow? They are all primary colors, and contrast well to the human eye.
The logo most likely to: be used as a dart board by Apple fanboys.