"Cannot modify header information - headers already sent by (output started at codefolder\modules\user\user.module:1)" i am using drupal 6, mysql 5.5 on apache 2.2 it is working fine.
but when i migrated my code on xampp1.7.3(contain apache2.2 and mysql 5.5).
it gives issue and error.


  warning: Cannot modify header information - headers already sent by (output started at codefolder\modules\user\user.module:1) in codefolder\includes\session.inc on line 99.
  warning: session_regenerate_id() [function.session-regenerate-id]: Cannot regenerate session id - headers already sent in codefolder\includes\session.inc on line 102.
   Blockquote


and after login pages containing urls like localhost/node/55 shows white pages. while without login it works fine.
while searching on google i found this issue might be due to header but i haven't used any header redirect in my  .tpl.php files.
Check that you don't have any ending php tags at the end of your php files. Sometimes whitespace (spaces, tabs, newlines) will sneak in at the end of the file after the tag and cause php go into html mode and send the headers too early.