Appearance Settings on QuickTime Pro: Note for Mac users: the following instructions are only valid for QuickTime 7 Pro. On QuickTime Pro, you can change the Horizontal and vertical placement, Width and Height of the subtitle pane by going to Window -> Show Movie Properties.
Seems like soft subtitling is most often done with MKV files these days (at least, that's what I always see), and if you're willing to go that route yes, there is Windows software available. I can't tell you WHAT software, but it does exist. On the mac, have a look at Miyu.it can add subtitles to any file that Quicktime can open, and then export to a variety of formats.
I've used it successfully on a couple of projects, it's free, and once you get used to the interface it's pretty easy to use.