System requirements really depend on what you want to do. It depends on many factors. Will you be doing transcoding, recording ? Call volume ? Those are a few off the top of my head. From what I know of there was one realse by some one (not digium) a long time ago that allowed you to run asterisk on windows. This is really out dated. If you must run asterisk on a windows machine you can look in to using vmware. I would not recomend it as it will eat up a lot of resources. It’s worth it getting “dirty” with linux. Three years ago I knew nothing about Linux and now I make living off it. You will be supprised at how much you can pick up.
AsteriskNOW is a GUI that was designed for asterisk. It was created to make things easier for people that wanted to create a PBX real fast. At this time I would vote against using it. There seems to be a lot of “issues” with it. Even though it is out of Beta (or so I believe), I would give it more time before using it in a production enviroment. However if you are looking to just get your feet wet, it’s a great way to start.