Ok, I'm not sure why Dropbox isn't the perfect solution. It absolutely does "mirror the data files between the 2 computers." The files are stored both "in the cloud" and also locally on both (or more) PCs. They are sync'ed up pretty much in real time. Your desktop doesn't have to be on and you don't have to log on. Keep your files and folders within the dropbox folder and you don't have to do anything special at all, not even copy anything. You also get a 30 day running revision history on all your files. If you can fit within their 2 GB limit (plus more for referring friends), it should be the perfect free tool.