Microsoft Corporation (NASDAQ: MSFT) has two main cash cows at this moment: Windows and Office. Together, those two pieces of software (in all their iterations and versions) bring in the lion's share of revenue to the software maker. We're talking about tens of billions per fiscal year. Looking around the globe, there are free alternatives to both of those software franchises, though, which makes one think: why doesn't everyone use the freebies and stop paying for Microsoft's two big products?

The answer is bit complicated, so we'll attack the "Windows" question first. Microsoft licenses out the Windows operating system to almost all computer makers who then install it on the PCs they sell to the public and businesses (and everyone else). No customer probably told Dell Inc. (NASDAQ: DELL) or Hewlett-Packard Company (NYSE: HPQ) that they wanted Windows -- there simply is no choice when buying a PC. Recent advances by Dell to make the free Ubuntu Linux operating system available on some PCs has happened, sure. But, the majority of the public just wants what they are used to, which is Windows. People love change, right? Wrong. There are then many (many) versions of Windows for servers and workstations that are sold as well.

Let's turn to Microsoft Office. Again, it brings in a ton of revenue to Redmond, even though there are full office productivity suites that compete with it. My guess is that many of you can't name one though -- and that is what Microsoft counts on, which is branding power. The full-featured OpenOffice productivity suite is pretty darn impressive (I've used it), and it's completely free and interoperable with Microsoft Office (not sure about Office 2007). Why don't more people use it then? Lack of knowing it exists is a possibility, and having to download the suite or order it for about $10 on CD may be obstacles. Plus, you can't find it on Best Buy shelves. If customers start becoming smarter and find out out about OpenOffice or even Google Inc. (NASDAQ: GOOG)'s Google Apps, could a large piece of Microsoft's kingdom come crashing down?

[Disclosure: I own MSFT shares as of 5-29-07]

I remember when I first got my hands on a copy of Microsoft Word. After years on WordPerfect, the creative soul in me was drunk on the pretty way you could view pages as pages, not words separated by dotted lines, the user-friendly layout, and oh, the fonts. But now, I get my kicks off in-line editing and next-gen blog software and AJAX (it's the wave of the future, dontcha know).

According to Jason Calacanis, CEO of Weblogs, Inc. and attendee at the D conference, Bill Gates is still in love with his software product. Jason writes that Bill "demoed a very cool version of Office that had a very clean -- but still packed -- GUI. One thing I noticed while the demo was going on was  that Gates was smiling like a kid on Christmas. As the audience oooed and ahhhhed you could see him get more excited and smile more. I thought to myself, 'wow, this guys been building the same pieces of software for over 20 years and he is still excited about it -- that's pretty cool.' "

Jason's other big takeaway: Microsoft is about to create "storage in the sky" for your MS documents. Now that would be truly useful and a worthy rival to Gmail and, if it wasn't imitated by Steve Jobs in a hurry, an Apple killer. In the world where everyone's mobile, you can't keep your data on one machine.