Netscape wa giving its browser away for free too. I remember I was downloading it from netscape.com without paying. What made Microsoft subject to an antitrust investigation was bundling IE with Windows. MS lost the case but too late for Netscape.

To answer another reply to your post, that's also the difference with Linus distributing Linux for free. No bundling. Furthermore there is the open source vs closed source issue which is no small difference too.

Ask Microsoft how they feel about Linus giving Linux away.