WorldWide Telescope from Microsoft Research may be of interest to some.

"The WorldWide Telescope (WWT) is a Web 2.0 visualization software environment that enables your computer to function as a virtual telescopeóbringing together imagery from the best ground and space-based telescopes in the world for a seamless exploration of the universe."

The setup file is 20.8Mb and system requirements include Windows XP SP2 (minimum) or Vista, and ,NET Framework 2.0