This happened to me: I set up a new iTunes Music Store account, and I got various kinds of errors every time I tried to download a purchased song. To make a long story short, iTunes gave me an error when I tried to deauthorize my unauthorized computer. So I decided, "Hey this is UNIX! I bet I can find the account info and delete it."
ITMS account info is stored in your user's Library -> Preferences folder, in a file named com.apple.internetconfig.plist. Inside the XML file, one of the sets of data starts with <key>Helper•itms</key>; look between <data> and </data> in this section. There should be several lines of garbage, with parts of it being hex. Replace the hex-garbage with AAEBcw==. When done, your iTunes account entry should look like this:
ITMS account info is stored in your user's Library -> Preferences folder, in a file named com.apple.internetconfig.plist. Inside the XML file, one of the sets of data starts with <key>Helper•itms</key>; look between <data> and </data> in this section. There should be several lines of garbage, with parts of it being hex. Replace the hex-garbage with AAEBcw==. When done, your iTunes account entry should look like this:
<key>Helper•itms</key>
<dict>
<key>ic-data</key>
<data>
AAEBcw==
</data>
</dict>
Now try opening iTunes and browsing the store.
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