Please explain "download the most"
What operating system?
I have a Mac I lightroom 21 which is installed, but can it Lightroom installed on another machine (Windows)
You can download Lightroom 2.1 for Windows here: http://www.adobe.com/support/downloads/product.jsp?product=113&platform=Windows
You will need your serial number for Lightroom 2 when you install it on Windows.