Some have asked in the past about building for 64-bit using previous IDEs such as Visual Studio 2003 and Visual C++ 6.0. Even though this is not a supported scenario, it is possible. Please follow this
link in order to get instructions on how to accomplish this.
I cannot stress this enough: if you need to build 64-bit binaries, please use Visual Studio 2005. By doing so, you know that you are dealing with a supported scenario, are most likely to find answers to your problems online, and will make sure you are using the most stable versions of several programming libraries such as ATL, MFC, etc.