How to downpatch to 3.3.5a (12340)

Step 1: If you have the latest 4.0.1 patch, then you need to reinstall the game, that's the only way to get back to the 3.3.5a patch.

Step 2: After installing WoW: WOTLK then you need to start downloading the game patches.

Step 3: You need to download each patch, and install it manually, here is the link.
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enUS client: http://us.blizzard.com/support/article.xml?locale=en_US&articleId=21079
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Every other client: http://www.strategyinformer.com/pc/patches/worldofwarcraft/patch.html

Step 4: Patch up to 3.3.5a (Build 12340)

That is all you have to do to be able to play! Please remember to Vote, and have a good day at GorillaWoW!