First, I am not crazy about a platform, neither think one of them is to be trashed; in fact, I was born with DOS, then UNIX at my firt work, then Windows (at a 2000 pc installation and even an MCP) and actually getting back to learn how Linux can really help.
I agree most with OZ, others pointed to "linux hard die" people but they really are MS die hard, that without really doing research and testing of alternatives think they can predict Linux will not be able to compete with Windows. Some of the people, like me, that want this to be clear have to install the product, test it, and compare it with Windows. And this is an ongoing process, 6 years ago I co-decided to implement a new network that will grow with Windows, my decision consider Linux and discard it promptly; if I have to do the same job with the actual conditions, and have to deal with license cost, security related issue and stability, there is a really good chance that Linux be elected.
And this, despite that here in Guatemala we do not have yet a strong presence of Novell or RedHat, but web and local people support could do the job, meanwhile Novell or Redhat stablishes local support/offices to compete with MS marketing people (this is its rock solid area).
Then again, suppose we go with Windows because of an application that requires it, you can be sure Open Office would be an excelent oponent to Office, Firefox to IE, 7zip to Winzip, Thunderbird to Outlook, and this would have lower costs and perhaps let us buy better hardware.
I alread forgot, the not-to-smart point that MS is lower in licenses costs or that these costs (OEM) are not important to consider, I just can say that this means that you apply "costs as a state of mind", because, since you have never really do your numbers, you are assuming that your MS is costing less than alternatives.
Then again, there is no universal solution, they all try to convince/confunse with biased articles (see MS Know the facts, Novell's Truth Untold and Redhat's Truth Happens), but you need to apply your own analisis, once you have done your homework and research, test and do the numbers for YOUR company or project.
What you have paid for your Windows PC (9x, NT, XP) will be paid higher when you are asked to migrate to next version, and maybe your apps will have to be migrated, and your hardware, and your knowledge ... and your provider would have retired your support and you will not have the access to the source code or a comunity to back it up...