Microsoft is allowing anyone to download and install a free copy of the operating system so that it can be tested on wide range of machines. The company still hasn't announced the date of the final commercial version though it's widely expected to be out by the end of this year.

What you download is an ISO file which must be burned to a DVD before you can install it on a machine so by sure your PC has a DVD burner and you have a blank DVD handy. Microsoft says that your machine will need a 1 Ghz or faster CPU, at least 1 GB of RAM (I recommended at least 2 GB) and at least 16 GB available disk space (more is better). The 64-bit version has higher requirements. 

You can install Windows 7 over Vista but not earlier versions of Windows. For best performance, I recommend a "clean install" which requires that you re-install all your software when you're done. In theory this can be done without destroying the data on your disk, but I would never dream of installing an operating system without first backing up all of my data.

Libraries are virtual folders that, by default, provide access to documents, photos and other media files without having to navigate through a hierarchy of folders. You can use this to add links to additional folders on your machine, a network server or another machine on the network. For instructions on how to use libraries see this post on Windows Live. Article according to CNET News.