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I would list a number of possible problems and if I'm wrong maybe someone could correct me.
1. Which copy did you use? A *.tar.gz or *.zip version?
2. Which program, utility, whatever did you use to extract the file?
3. How did you upload your files? Which FTP program did you use?
4. Are the settings in mainfile.php the correct ones? They should be if you ran the install script but in any case, check it out again.
My guess is that it has something to do with a corrupted file. Try uploading again, run the install script and tell us if you get the same error.