Compare the fake application on the left with the real thing on the right, and note the differences:
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Immediately, we can see that the real thing has a rounded curve at the top - the fake is blocky, and looks like a regular Windows application box. There's an "Inbox" link at the top when you start up the fake application - there isn't a link like that when firing up Google Talk for the first time. The Username / Password box is much lower down on the fake application, and (again) the real "Sign In" button is curved on the real application. Finally, you'll see "Forgot your account / Don't have an account" on the genuine Google Talk program - not so on the fake.
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