Hi Luis,
what u can do with the old comp is delete all saved flights from FSX and then try the CS 757...this problem is overlooked many times. I'm glad u have tit running in u'r second comp, but having it run in u'r primary is something quite different..see if that helps in any way..another thing u can do is turn off firewall and anti-virus while u download the addons and turn them back on after the download...hope it works out for u.