By accident I stumbled about an issue that (older 32bit??) Windows systems might limiting your outbound TCP connections. I'm not an expert on this field but I guess its worth to check if you are affected by this. This should be especially true for all home-PC users as they are using 32bit OS often times. I'm personally not affected by this and tested it on 'Win 2008 Server R2 (standard)' and 'Win 7 64bit (ultimate)'.
This little tool will check if you are limited and give you a message if nothing can be optimized:
FAQ about tool and issue:
Please be aware of the fact, that I literally don't have a clue about this topic and it is possible that there isn't any system affected for the case that Mircosoft have patched all OS. I just thought that it might worth to share this for the case that your system is thwarted due to this.