If you really want "plain old C", which would be considered obsolete for most applications, then you can download an old C compiler from the web and buy a book such as "The C Programming Language" by Kernighan and Ritchie.
But since you posted your question in the Microsoft Training and Certification forum, I presume that you are looking for one of the newer versions of the language, as supported by the Micorosoft development tools, such as C# or C++.
To set up your development environment, you can download one of the Express editions of Visual Studio, which are free:
http://www.microsoft.com/visualstudio/eng#products/visual-studio-express-products
Note that there are several variants of Visual Studio Express. The one you choose depends on the type of programs that you want to develop (desktop, web, ...). The programming language itself is the same one in all editions.
