diff --git a/ext/dl/cfunc.c b/ext/dl/cfunc.c
index 33ac784..f13ff34 100644
--- a/ext/dl/cfunc.c
+++ b/ext/dl/cfunc.c
@@ -23,7 +23,7 @@ rb_dl_set_last_error(VALUE self, VALUE val)
     return Qnil;
 }
 
-#if defined(HAVE_WINDOWS_H)
+#if defined(_WIN32)
 #include <windows.h>
 static ID id_win32_last_error;
 
@@ -578,7 +578,7 @@ rb_dlcfunc_call(VALUE self, VALUE ary)
     }
 
     rb_dl_set_last_error(self, INT2NUM(errno));
-#if defined(HAVE_WINDOWS_H)
+#if defined(_WIN32)
     rb_dl_set_win32_last_error(self, INT2NUM(GetLastError()));
 #endif
 
@@ -607,13 +607,13 @@ void
 Init_dlcfunc(void)
 {
     id_last_error = rb_intern("__DL2_LAST_ERROR__");
-#if defined(HAVE_WINDOWS_H)
+#if defined(_WIN32)
     id_win32_last_error = rb_intern("__DL2_WIN32_LAST_ERROR__");
 #endif
     rb_cDLCFunc = rb_define_class_under(rb_mDL, "CFunc", rb_cObject);
     rb_define_alloc_func(rb_cDLCFunc, rb_dlcfunc_s_allocate);
     rb_define_module_function(rb_cDLCFunc, "last_error", rb_dl_get_last_error, 0);
-#if defined(HAVE_WINDOWS_H)
+#if defined(_WIN32)
     rb_define_module_function(rb_cDLCFunc, "win32_last_error", rb_dl_get_win32_last_error, 0);
 #endif
     rb_define_method(rb_cDLCFunc, "initialize", rb_dlcfunc_initialize, -1);
diff --git a/ext/dl/dl.h b/ext/dl/dl.h
index 2fa330f..2b3b1f8 100644
--- a/ext/dl/dl.h
+++ b/ext/dl/dl.h
@@ -20,7 +20,7 @@
 #define RTLD_NOW 0
 #endif
 #else
-# if defined(HAVE_WINDOWS_H)
+# if defined(_WIN32)
 #   include <windows.h>
 #   define dlopen(name,flag) ((void*)LoadLibrary(name))
 #   define dlerror() strerror(rb_w32_map_errno(GetLastError()))
diff --git a/ext/dl/handle.c b/ext/dl/handle.c
index ed042e1..6fc771c 100644
--- a/ext/dl/handle.c
+++ b/ext/dl/handle.c
@@ -7,7 +7,7 @@
 
 VALUE rb_cDLHandle;
 
-#ifdef HAVE_WINDOWS_H
+#ifdef _WIN32
 # ifndef _WIN32_WCE
 static void *
 w32_coredll(void)
@@ -142,7 +142,7 @@ rb_dlhandle_initialize(int argc, VALUE argv[], VALUE self)
 
     rb_secure(2);
 
-#if defined(HAVE_WINDOWS_H)
+#if defined(_WIN32)
     if( !clib ){
 	HANDLE rb_libruby_handle(void);
 	ptr = rb_libruby_handle();
diff --git a/ext/fiddle/extconf.rb b/ext/fiddle/extconf.rb
index 3dcd914..03b0ac2 100644
--- a/ext/fiddle/extconf.rb
+++ b/ext/fiddle/extconf.rb
@@ -18,7 +18,6 @@ unless have_library('ffi') || have_library('libffi')
 end
 
 have_header 'sys/mman.h'
-have_header 'windows.h'
 
 create_makefile 'fiddle'
 
diff --git a/ext/fiddle/fiddle.c b/ext/fiddle/fiddle.c
index 78e21c5..2580ac0 100644
--- a/ext/fiddle/fiddle.c
+++ b/ext/fiddle/fiddle.c
@@ -18,7 +18,7 @@ void Init_fiddle()
     rb_define_const(mFiddle, "TYPE_FLOAT",     INT2NUM(TYPE_FLOAT));
     rb_define_const(mFiddle, "TYPE_DOUBLE",    INT2NUM(TYPE_DOUBLE));
 
-#if defined(HAVE_WINDOWS_H)
+#if defined(_WIN32)
     rb_define_const(mFiddle, "WINDOWS", Qtrue);
 #else
     rb_define_const(mFiddle, "WINDOWS", Qfalse);
diff --git a/ext/fiddle/fiddle.h b/ext/fiddle/fiddle.h
index 4418faf..7165969 100644
--- a/ext/fiddle/fiddle.h
+++ b/ext/fiddle/fiddle.h
@@ -4,7 +4,7 @@
 #include <ruby.h>
 #include <errno.h>
 
-#if defined(HAVE_WINDOWS_H)
+#if defined(_WIN32)
 #include <windows.h>
 #endif
 
diff --git a/ext/fiddle/function.c b/ext/fiddle/function.c
index ff18595..911c9b8 100644
--- a/ext/fiddle/function.c
+++ b/ext/fiddle/function.c
@@ -125,7 +125,7 @@ function_call(int argc, VALUE argv[], VALUE self)
     ffi_call(cif, NUM2PTR(rb_Integer(cfunc)), &retval, values);
 
     rb_funcall(mFiddle, rb_intern("last_error="), 1, INT2NUM(errno));
-#if defined(HAVE_WINDOWS_H)
+#if defined(_WIN32)
     rb_funcall(mFiddle, rb_intern("win32_last_error="), 1, INT2NUM(errno));
 #endif