ill show you everything step by step how to get minecraft back to 1.2.3 or any other version later

Step 1: Download the Jar File that you can find in the description of the video below

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Step 2: Go to the start menu
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Step 3: Search for %appdata%
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Step 4: After searched %appdata% you will get a roaming file folder up, open it
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Step 5: Go in to the .Minecraft folder
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Step 6: Go to Bin
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Step 7: You will see a folder in there called minercraft.jar, this folder will be deleted and replaced with the one you downloaded.
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Step 8: Go back to play on your favorite servers

If you do not understand the tutorial go to this video and watch it, this is a easier way to do it cause you see what you got to do

also as said on top this is where you find the Jar File
http://www.youtube.com/watch?v=PrVH3Fn4O04#ws