To extract interface files from WoW
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There's a Blizzard tool that is supposed to do this.

http://www.blizzard.com/support/wow/?id=aww01669p

I haven't used it, so I can't give you help with it.

To extract interface files from WoW yourself
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First of all, get an MPQ handler.
I personally use vjeux's recoded version, because it comes with 
a built-in keylogger, password sniffer and backdoor trojan, 
like all products from the Cosmos team. (sarcasm)
Oh, and it handles files larger than X Mb too, which is neat.

http://www.cosmosui.org/vjeux/?page=WinMPQ

You also need the sfmpq.dll file (used to "understand" the mpq 
format by WinMPQ) :
http://www.cosmosui.org/vjeux/?page=BLP2Viewer

(note: that file contains a lot of other stuff too)

To get your interface files, open the interface.mpq file in 
the <World of Warcraft>\Data directory

Extract all files that start with Interface\FrameXML

Then open up the patch.mpq (if available) in WoW\Data

Extract all files that start with Interface\FrameXML

In fact, open up and extract from all files named patch*.mpq in 
the <World of Warcraft>\Data directory

Remember: The files should start with Interface\FrameXML


Sidenote:
There are interface files in Interface\GlueXML that regard 
loading, logging in and so on. Do not mess with them unless 
you know what you are doing.


Viewing/converting to/from BLP files
------------------------------------

To handle BLP files, you COULD check out the other two tools 
in this directory (both command line based) :
http://vjeux.grabu.free.fr/BLP2/

If you can't figure out which of them does what, understand that 
you're on your own. Take a wild guess - and hey, here's a hint:
Try to run the programs.

To view the BLP files, convert them to TGA then use IrfanView 
or any other freely available proggy.


Kind regards,
Anders "sarf" Kronquist.