Have you tried contacting [email protected]? I ask only because if there are network problems or problems with your key/account/etc. they would be the most likely to be able to help.
Do you have any firewall software running? While the regular Windows Firewall doesn't block connections from your computer, other software (like ZoneAlarm and probably software from Norton or others) can. In that case it could block your connection.
Just some thoughts, it's been so long since I registered Sins that I don't even remember the process you're going through so sorry I don't have anything more useful to add 