Porting a WIN32 Project to x64

This is a WIN32 DLL project that I need to build as 64 bit DLL.  I ported the project per topic:

http://msdn.microsoft.com/en-us/library/vstudio/9yb4317s(v=vs.100).aspx

but when I build the DLL for x64 it gives the following error:

c:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\atlmfc\include\atlcore.h(35): fatal error C1189: #error :  This file requires _WIN32_WINNT to be #defined at least to 0x0403. Value 0x0501 or higher is recommended.

How do I get around this?