When trying to reinstall Magento once I failed earlier installation I got an error message “Magento has been downloaded earlier”. I had deleted all the earlier installation file and cleared the database too after running downloader.php which I had downloaded from Magento.com. I uploaded downloader file to next location still I got same error message of Magento has been downloaded earlier.
As a last resort, I cleared all cache, cookies and ran the same downloader.php. Sparky, it worked. So, if you get the same error message try clearing browser’s cache, cookies and run downloader.php again.