TortoiseSVN is a really easy to use Revision control / version control / source control software for Windows. It is based on Subversion. TortoiseSVN provides a nice and easy user interface for Subversion.
It is developed under the GPL. Which means it is completely free, including the source code. But just in case you don't know the GPL too well: you can use TortoiseSVN to develop commercial applications or just use it in your company without any restrictions.