-#define GSSEAP_THREAD_ONCE INIT_ONCE
-#define GSSEAP_ONCE_CALLBACK(cb) BOOL CALLBACK cb(PINIT_ONCE InitOnce, PVOID Parameter, PVOID *Context)
-#define GSSEAP_ONCE(o, i) InitOnceExecuteOnce((o), (i), NULL, NULL)
-#define GSSEAP_ONCE_INITIALIZER INIT_ONCE_STATIC_INIT
+/* Once is handled separately because INIT_ONCE is only on Vista and above */
+#define GSSEAP_ONCE_LEAVE
+#define GSSEAP_ONCE_CALLBACK(cb) void CALLBACK cb(void)