Starting with version 3.0, all MFC classes are "thread safe" and require the multi-threaded run-time libraries to link successfully. Many people try to use the single-threaded run-time libraries because they assume these libraries are needed to enable the application to run in Win32s. This is not the case. MFC versions 3.0 and later will use a single thread, so as long as you are not creating additional threads in the application, the application will run under Win32s.
To avoid these unresolved external errors, do not set the Project Settings to Single-Threaded for an MFC version 3.0 or later application. This setting can be changed by doing the following:
On Visual C 2.x and 5.0:
Select the PROJECT menu (and continue Steps 2 through 5 below).
On Visual C 4.x:
Select the Build menu.
Select the SETTINGS... option.
Select the tab, C/C++.
Select CODE GENERATION on the Category list box.
Finally, make a selection other than SINGLE-THREADED on the Use Run Time Library list box.