MinGW does not offer 64 bits at the present time, and to my knowledge, there is no other free compiler for Windows that does (although this may be wrong... possibly Cygwin does?).
The Intel compiler and the professional version of the Microsoft one should support 64 bit, but they cost money, of course. It is not only about the size of a pointer, there is an entirely different ABI and everything for 64 bits (bigger register set, wider registers, other calling conventions,...).
For the near future, I would not hope for a free 64 bit compiler for Windows (hopefully I'm wrong!).