In the spirit of recreating this error I sat down and played with blocking certain Blizzard domain names using my /etc/hosts file. Just mapped addresses to 127.0.0.1
I've found that when you block :
ak.worldofwarcraft.com.edgesuite.net = you get the "Failed to download information about the next patch" error.
enUS.patch.battle.net = us.patch.battle.net (canonical) = you get the "Launcher cannot obtain patching information" error
The first address uses port 80 to connect while the 2nd more relevant one use port 1119 to connect.
So another avenue would be to check if port 119 to the address enUS.patch.battle.net. I would say try telnet but I hear that's not a good idea on windows.
Also portforward.com can be used to help configure things to forward ports :)
EDIT One of my PC Techie friends said to try http://www.canyouseeme.org/ to see if your 1119 port is open or not.