One of my favourite websites is NASA's excellent Astronomy Picture of the Day (APOD). Every day, APOD features an outstanding photograph or image of some aspect of "our fascinating universe". A succinct but informative explanation compliments each picture.
There is also a downloadable version of APOD available that automatically downloads a new astronomy picture every day and displays it on your computer desktop. The desktop version includes the image explanation, which is heavily hyperlinked for those seeking more information. It is therefore educational as well as aesthetically pleasing.
The APOD software is free and "as is". However, I've been running it for several weeks on my Windows XP system with nary a hitch.