The License Header tool is a simple application which does one thing- Add a chunk of text (typically your license text) to the header of every file in a directory. The file types to add the text to are configured in a list, and the edited files are then copied to a second directory, thus the original files are not overwritten. You can also recurse into all sub directories and process files in them as well.
The tool is written in .Net 4.0, using WPF, so please make sure you have the latest .Net 4.0+ framework installed.
Download the free software from here: License Header Tool