Index: src/ui/system.c
===================================================================
--- src/ui/system.c	(revision 7852)
+++ src/ui/system.c	(working copy)
@@ -317,6 +317,15 @@
 		return luaL_error(L, "fclose: %s", strerror(errno));
 	}
 
+/* windows systems must delete old file first */
+#if defined(WIN32)
+	if (_access_s(fname, 0) == 0) {
+		if (remove(fname) != 0) {
+			return luaL_error(L, "remove old file: %s", strerror(errno));
+		}
+	}
+#endif
+
 	if (rename(tname, fname) != 0) {
 		return luaL_error(L, "rename: %s", strerror(errno));
 	}