If you are recieving this message Internal Client error: session.minecraft.net Then the problem is not with your Minecraft client, nor the minecraft server. It's your DNS server.
This will work: Start from your root directory (usually drive C) then go to windows -> System32 -> drivers -> etc
open the "hosts" file using notepad read the paragraph to help you understand the file then enter this on one line by its self:
IF RUNNING WINDOWS Vista or 7 Go to startmenu Search for notepad Run notepad as Administrator Open the "hosts" file at windows -> System32 -> drivers -> etc
now you should see a bunch of # symbols down the side,
go all the way to the bottom and on the last line that doesnt have a # symbol paste the thing below, EXACTLY, NO # SIGN, JUST THIS ON A NEW LINE.
Paste the thing above save and close. Then you are done. and Enjoy all those servers and no it doesnt harm your computer, it just means your computer thought that minecraft shouldnt be allowed to access the internet because it doesnt understand the fun of minecraft.