I have gone through a similar problem.
Have you ever installed other blog tools(even wordpress, itself) on your default root or subdirectory? If so, that can make index.php or .htaccess file modify automatically.
Then, what you need to do are…
1.Delete all files that you have uploaded and .htaccess file(especially, .htaccess file is a hidden file. So you need to turn on’show hidden files’ option of your ftp program)
2.Delete databases related to wordpress of MySQL.(You can distinguish them with a identifier, starting as “wp_”)
3.Reinstall wordpress.
In my case, those could solve this problem. If you don’t know how to do 2nd thing, ask your web host company.
I’m not a Eng native speaker, I hope you can understand what i wrote.
