How to I debug “Cannot modify header information

I have successfully installed WordPress on Linux and the default blog shows upp.
However, when I try to log in, the last step in the install process, I get

Warning: Cannot modify header information – headers already sent by (output started at /home/frantze/public_html/wp/wp-config.php:34) in /home/frantze/public_html/wp/wp-login.php on line 123

and 7 other similar lines.
I have red the forum and I know that this is a pretty common problem.
I dropped the database, deleted the entire wordpress directory, uploaded the files a second time, and I still get the error. So it can hardly be a file corruption problem.
So how do I debug this show stopper?
I have checked the wpp-config.php file and there is no crud before or after the only <?php tag and endtag there.
I have not touched the php-login.php file at all, and it looks perfectly OK. There is nothing that catches the eye on line 123 in wp-login.php. My file permissions are 755, which ought to be correct.
Any suggestions