Quoting from this site:
Almost three years ago the naval systems arm of major UK defence contractor BAE Systems took the decision to standardise future development on Microsoft Windows. an immediate effect was to commit BAE’s joint venture CMS subsidiary, AMS, who specialise in naval Combat Management Systems, to implementing a Windows 2000-based CMS system for the new Type 45 Destroyer. But this prompted strong internal opposition from some of AMS’ engineers, who had a sound background in Unix and who had, despite resource starvation and a companywide policy to standardise on Windows, been investigating open source alternatives as a foundation for future combat systems.
… Click the link, it’s scary.