I'm not sure, but somehow I doubt it is the firewall, as if it was, it would either deny the BT connection all together, or let it pass through and perform like normal. Makre sure that the only that changed between your old and new machine is the operating system. Who knows what could be affecting your download rate. A good way to narrow down the possibilities is to download a file via http and if that max's out much higher than your BT download, then you know it's an issue either with BT client or your ports that it goes through.