Total Cost of Ownership (TCO) depends so much on business (or personal) requirements that it just does not make sense to make generalizations. A key decision point is often: 'we have to run this off the shelf software, and only a Windows version is available'. This is commonly the case in banks, stores, etc that purchase complete solutions that include Windows clients.
For me personally, TCO has more to do with how many hours a month do I need to spend to administer the computers that I need for my consulting business. Admin time is non-billable time. I use Mac OS X, Ubuntu (Debian derived) Linux, and Windows. I find that the admin time required for OS X and Ubuntu is far less that for Windows - way less. Still, for some jobs I need to use Windows, so I live with higher administration overhead.
A customer of mine has a large work group in India (with just a few of us in the US) and is looking to move away from a Windows desktop. I think that the OpenOffice.org support for Microsoft Office file formats is so good now that they should be able to do this, but it would be naive to not expect human/labor costs in the switch. The big win is long term: not dealing with software license keys, no pressure of forced updates when current systems meet operational requirements, no ongoing software licensing costs, etc. One great thing about Linux is that it tends to run well on lower cost hardware: in 1992 I ran Linux, X windows, and C++ development tools on a used laptop (5 MBytes of RAM, really small disk) and today, I get surprisingly good use out of a $200 Chinese manufactured PC (I did add more memory to it, so it was really a $250 PC). Because I use this PC a lot (with Ubuntu Linux on it) running OpenOffice.org, and IntelliJ Java IDE, I am occasionally tempted to invest a little money and some (non-billable) admin time replacing it with a faster box - but, with Linux, even this low end PC is so useful, I don't feel like investing the time to upgrade the hardware.
My personal preference is to run OS X, but I feel like there are long term advantages to mastering the Linux platform, both server side and on the desktop. So for strategic reasons, I find myself booting Linux more often.