YouTube Downloader

I need to download an app thnat will allow me to download YouTube video's then convert to Windows Media Player. There seem to be a lot available for free but I am hesitant in downloading anything from the web on to my (still new) machine (Mac OS X 10.7.4) Has anyone any recommendations or for that matter downloaders to avoid? I really dont want to cause myself any problems that I wont feel able to sort out (I'm still new to all things Mac!)