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It will be very difficult for anyone to help you without more information.
Can you post a screenshot of the dialog you are encountering?
I would be happy to try to replicate your situation. Please post a link to the webpage you downloaded the installer from.
What version of AutoCAD? (2010, 2011, other?) Was it plain (vanilla) AutoCAD, or a vertical product (such as Mechanical, Architectural, Civil, etc.)?
What operating system? (Windows XP, Vista, 7, other?)
Have you previously had another AutoCAD trial version installed?