At least at present there are a thousand or more Windows systems in business for every Linux system. That may change in the future, but I wouldn't expect it any time in the next 3-5 years.
At present, there are roughly 12,000 Windows systems for every Unix system in business. I really don't expect to see Unix make any real headway toward reducing that ratio, although Darwin might eventually prove me wrong.
I would work with both Windoze and Linux if I were you, but I'd expect the Windoze to put food on the table for the forseeable future, simply because that is what businesses actually use.
I'd pick several of the server level products, and try to focus on them. SQL Server is a good choice, Exchange, and IIS are also good. These are the products that are likely to be in high demand for technical support, which means that they are what are likely to make you the "big bucks" if you know how to make them behave.
From the standpoint of which languages and tools are likely to be useful, I'd recommend C++ because it offers you a lot of insight into compilers in general, object orientation, general scripting, etc. PHP is also a good choice because it is a powerful scripting language, and has a strong presence on the web. I've got a personal weakness for Perl, but that is probably because I've used it for so long and it is so blasted elegant!
-PatP