@FriendlyFireGuy - If this problem persists following the suggestions above, a quick fix would be to open a command prompt using 'CMD' in the RUN menu, and typing IPCONFIG /FLUSHDNS.
This will reset and remove any DNS based errors you are seeing..
P.S, I've seen a similair issue with the google search engine on Firefox, it does not let me reach the same sites you mentioned until I run the above command.