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The ClearType Tuner only works on Windows Vista and XP, and is available as a web-based tool here. You will be required to choose which is the clearer text from a variety of options a couple of times, and then - hopefully - text should be clearer on your screen. Windows XP users can also download a Control Panel version from the Microsoft XP Power Toys page.
If you are a Vista user and wish to switch off ClearType check out Diana Huggin's instructions on the excellent Lockergnome site.