Redirect to HTTPS for all except Windows XP

ssl.conf

server {
	...
	
	listen 443 ssl;

	listen 80;

	if ($server_port = 80) {
		set $xp A;
	}		
	if ($http_user_agent !~ "Windows NT 5\.[12]") {
		set $xp "${xp}B";
	}
	if ($xp = AB) {
		return 301 https://$host$request_uri;
	}
}