Partial fix to [ Bug #128685 ] popen on Win9x isnt smart enough about finding w9xpopen.exe.

"Partial" as the code uses sys.prefix in an attempt to locate 'w9xpopen.exe', but sys.prefix is not set if Python can't find it itself.  So this _still_ fails in Pythonwin, but I am committing the patch for 2 reasons:
* Embedded apps that set sys.prefix or use PYTHONHOME will work
* The exception raised on failure to find the executable is far more obvious

Modules/posixmodule.c

@@ -2289,7 +2289,7 @@ win32_popen4(PyObject *self, PyObject  *args)
 	return f;
 }
 
-static int
+static BOOL
 _PyPopenCreateProcess(char *cmdstring,
 		      HANDLE hStdin,
 		      HANDLE hStdout,
@@ -2299,7 +2299,7 @@ _PyPopenCreateProcess(char *cmdstring,
 	PROCESS_INFORMATION piProcInfo;
 	STARTUPINFO siStartInfo;
 	char *s1,*s2, *s3 = " /c ";
-	const char *szConsoleSpawn = "w9xpopen.exe \"";
+	const char *szConsoleSpawn = "w9xpopen.exe";
 	int i;
 	int x;
 
@@ -2321,22 +2321,55 @@ _PyPopenCreateProcess(char *cmdstring,
 			 * Oh gag, we're on Win9x. Use the workaround listed in
 			 * KB: Q150956
 			 */
-			char modulepath[256];
+			char modulepath[_MAX_PATH];
+			struct stat statinfo;
 			GetModuleFileName(NULL, modulepath, sizeof(modulepath));
 			for (i = x = 0; modulepath[i]; i++)
 				if (modulepath[i] == '\\')
 					x = i+1;
 			modulepath[x] = '\0';
+			/* Create the full-name to w9xpopen, so we can test it exists */
+			strncat(modulepath, 
+			        szConsoleSpawn, 
+			        (sizeof(modulepath)/sizeof(modulepath[0]))
+			               -strlen(modulepath));
+			if (stat(modulepath, &statinfo) != 0) {
+				/* Eeek - file-not-found - possibly an embedding 
+				   situation - see if we can locate it in sys.prefix 
+				*/
+				strncpy(modulepath, 
+				        Py_GetExecPrefix(), 
+				        sizeof(modulepath)/sizeof(modulepath[0]));
+				if (modulepath[strlen(modulepath)-1] != '\\')
+					strcat(modulepath, "\\");
+				strncat(modulepath, 
+				        szConsoleSpawn, 
+				        (sizeof(modulepath)/sizeof(modulepath[0]))
+				               -strlen(modulepath));
+				/* No where else to look - raise an easily identifiable
+				   error, rather than leaving Windows to report
+				   "file not found" - as the user is probably blissfully
+				   unaware this shim EXE is used, and it will confuse them.
+				   (well, it confused me for a while ;-)
+				*/
+				if (stat(modulepath, &statinfo) != 0) {
+					PyErr_Format(PyExc_RuntimeError, 
+					    "Can not locate '%s' which is needed "
+					    "for popen to work on this platform.",
+					    szConsoleSpawn);
+					return FALSE;
+				}
+			}
 			x = i + strlen(s3) + strlen(cmdstring) + 1 +
 				strlen(modulepath) + 
 				strlen(szConsoleSpawn) + 1;
+
 			s2 = (char *)_alloca(x);
 			ZeroMemory(s2, x);
 			sprintf(
 				s2,
-				"%s%s%s%s%s\"",
+				"%s \"%s%s%s\"",
 				modulepath,
-				szConsoleSpawn,
 				s1,
 				s3,
 				cmdstring);
@@ -2345,8 +2378,10 @@ _PyPopenCreateProcess(char *cmdstring,
 
 	/* Could be an else here to try cmd.exe / command.com in the path
 	   Now we'll just error out.. */
-	else
+	else {
-		return -1;
+		PyErr_SetString(PyExc_RuntimeError, "Can not locate a COMSPEC environment variable to use as the shell");
+		return FALSE;
+	}
   
 	ZeroMemory(&siStartInfo, sizeof(STARTUPINFO));
 	siStartInfo.cb = sizeof(STARTUPINFO);
@@ -2373,6 +2408,7 @@ _PyPopenCreateProcess(char *cmdstring,
 		*hProcess = piProcInfo.hProcess;
 		return TRUE;
 	}
+	win32_error("CreateProcess", NULL);
 	return FALSE;
 }
 
@@ -2564,7 +2600,7 @@ _PyPopen(char *cmdstring, int mode, int n)
 					    hChildStdoutWr,
 					    hChildStdoutWr,
 					    &hProcess))
-			 return win32_error("CreateProcess", NULL);
+			 return NULL;
 	 }
 	 else {
 		 if (!_PyPopenCreateProcess(cmdstring,
@@ -2572,7 +2608,7 @@ _PyPopen(char *cmdstring, int mode, int n)
 					    hChildStdoutWr,
 					    hChildStderrWr,
 					    &hProcess))
-			 return win32_error("CreateProcess", NULL);
+			 return NULL;
 	 }
 
 	 /*