From MSDN about the ASSERT macro:
In an MFC ISAPI application, an assertion in debug mode will bring up a modal dialog box (ASSERT dialog boxes are now modal by default); this will interrupt or hang the execution. To suppress modal assertion dialogs, add the following lines to your project source file (projectname.cpp):
// For custom assert and trace handling with WebDbg
#ifdef _DEBUG
CDebugReportHook g_ReportHook;
#endif
Once you have done this, you can use the WebDbg tool (WebDbg.exe) to see the assertions.