I had my pc and its Windows XP operating system reformatted and reinstalled (using this method: http://icrontic.com/articles/reformatting-the-right-way/3). But after restart, the "Error downloading operating system" appeared. How will I be able to fix this?

The best thing to do would be to do it again. Something has played up during the copying of files. You could spend ages trying to find the exact problem. As you have a clean install, just do it again.