Problem with Zen cart installation

Hi,
I tired to install Zen Cart (zen-cart-v1.3.9h-full-fileset-10262010) via my Filezilla FTP yesterday and everything went ok. However I didnt pay enoug attention what I chose for my admin username and so couldnt enter there. The easy way I thought to insall everything again and I deleted all the files from my Host. I copied back all the files again and I even could get the installation page of zen cart. However when I want to start installation it gives me the error below :
Server error!

The server encountered an internal error and was unable to complete your request.

Error message:
Premature end of script headers: index.php

If you think this is a server error, please contact the webmaster.

Error 500

I will appreciate if somebody can help me with that.
Thanks in advance

I would delete everything from the server and clear the database and make a fresh install. But I think you must make everything has been removed. Much quicker than trying to find a missing or corrupted file.

I would delete the index.php file that is there now and re-upload it.

For many various reasons, FTP uploads can fail and parts of a file do not get uploaded. So you just upload the file(s) again.

I had the same issue.

username: Admin (uppercase A)
password: admin (all lowercase)

worked

The new version has taken over my original site, Any thoughts on reversing it?

Thanks for the advices.
I deleted everything once more and followed the instructions of the installation. it worked now.