This tutorial is based on the newest game launcher, if you never got it yet you need to do that first!
Get the newest launcher here: http://www.minecraft.net/download.jsp
*Please note* The launcher is not the same as the game, the launcher is the part you see when you log in.

That version number, if you have that on v 1.4 carry on reading. What we want to do now it delete everything minecraft related on your computer, however don't do this straight away without saving your singleplayer worlds. I will show you that.
Here is what you want to do.
Open the game and click on "Texture Packs".


This is what you should see, what we want is ".minecraft" so click on that up top.

Mine could look different to yours, I installed some mods. However what we are after now is "saves".

Copy it by right clicking the folder copying it. Then pasting it somewhere safe you will remember it in. (Eg: My Documents)
Now we've done that we want to get rid of minecraft all together.

Click on Roaming and then you should come to this page. Right click the minecraft folder and then delete. Don't worry you've got your maps safe and sound.
Now we want to download the downgrade. Here is the url
--Removed--
Once you have downloaded that unzip it and open it up till you see ".minecraft"

Now copy that ".minecraft" folder over to the "Roaming".
Once that is done open your new .minecraft folder and run "Minecraft (2)".
Once you log in the old Mojang colour schemed logo will pop up and then this.

Click NOT NOW.

Once that is done you can click on multiplayer and play!
If you want your save files from the singleplayer go back to where you put your previous file named "saves" and take it back to the ".minecraft" folder and paste it in.
All done hope that helps. Any questions please leave it here and I will try and answer asap.
Also people could be up on the chat who are willing to help.
http://www.maxloader.nl/chat/blab.php