Stellarium is a complete Windows application that lets you explore the components of the night sky. Pan around the screen to control your virtual telescope to look at tens of thousands of planets and stars. The graphics are superb, the amount of information contained in the program is immense, and there's even a 300-page book included as a PDF file that will teach you everything you need to know about how to drive the program.
Stellarium can even be used to control a real telescope if you have one that's compatible, but it's still amazing fun if you don't.
The program is completely free from http://stellarium.org/ and the 64-bit version is a 160 MB download. It's malware-free according to the security software I use (it's too large for VirusTotal) and also Web of Trust.
On any computer, Stellarium is good. If you have a large screen, or even a projector, you're definitely in for a treat.
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