Is it possible to download Open Sans as available on Google Fonts to use on my desktop?
I agree with FRD to some extent. And if you want to take print out of some text in the same font, then you'll have to Drag and Drop the respective .ttf files to the Fonts Explorer, which you can find in the Control Panel.
I installed by simply opening the file and clicking on the installation button. But when I printed out the text on a paper, it was some other font. So dragging and dropping worked well for prints as well.
I want to use the font on Windows, but it seems that Google Fonts are for use on the web.