Last week I had to uninstall the firewall off my laptop and boot before some polluted rules stuck somewhere would clear out and let me download work again.... i.e. exiting might not be enough. It's not uncommon for a complete uninstall needed of AV/Antispam/FW to proof that they are in fact the source.

But, if you have the same firewall as knreed, follow his steps first (versions could still be different).