So today Punkbuster is down, so pretty much every PC player cannot join any servers! We have found a working fix and so will share it with you here.
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So step one, download this file! Its virus checked and clean.
Now when this has downloaded, open the ZIP file and then move the files inside to the pb folder inside your Battlefield 3 install directory. Overwrite any files that are already there.
Now you should be able to join servers again!