You've probably seen this item mentioned elsewhere, but it bears repeating on "Linux in Exile":
Switching an office to Linux or other free / open source software comes with its own hidden costs - training, conversion, new software, etc. But Microsoft fans often gloss over the costs to run third party tools and anti-virus software, to fill in the gaps in functionality and security missing from Windows.
Those costs only go up when security fails in Windows. As David Ottewell writes in the Manchester Evening News, taxpayers in Manchester £1.5 million to clean up a Cornficker worm that infested Manchester town hall. And the best part? £600,000 of that went towards consultants to help fix the problem, including experts from Microsoft.
Do me a favor: the next time someone tries to sell you the line that "Microsoft has a lower TCO than Linux", please remind them of failures like this one.