Well you probably cant limit it to one computer... however you could limit it to IP's. One active logged in user session / ip address. It would only be a problem if someone has a dynamic IP and it changes while downloading a large protected file, however the browser/downloader should just prompt a login. Most dynamic ip's wont change unless forced by the modem with a redial, that's how all the ones I have had are anyway.
It's an idea.