There are lots of free C compilers. In fact the oldest maintained and most widely used, GCC, is free. There are even several desktop front-ends (called IDEs or Integrated debugging editors) for it. Of course widely used means it's used for everything from supercomputers to smartphones. The desktop -- and in particular the Windows Desktop is not a focus of it and while it's the compiler for several IDEs it remains a command-line program.
http://gcc.gnu.org
My favorite for the Windows desktop is Digital Mars C. I happen to like the command line, which it uses: There is an IDE available for it for fifty bucks, but I mainly use Linux anyhow -- for ALL my computing.
http://www.digitalmars.com
Anyhow, Windows has Visual C++ Express, and there are other options so I'm linking to the three most reliable lists of free compilers below. Read the comments on all three pages then choose and try them out.
http://www.freebyte.com/programming/cpp/
http://www.thefreecountry.com/compilers/cpp.shtml
http://www.freeprogrammingresources.com/cppcomp.html