You have a very rare problem and you havent even posted your pc specs and details thus its hard to even take a guess what could it be, but as I said it sounds like you have something loaded in memory that interferes with proper operation of the system by which gamespy lobby/mp verifies files.
Aside from making sure you fully removed the game and profile and reinstalling from steam again there is not much anyone can tell you without specific information about your pc.
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