I have a file, which starts <?php session_start();?>, and it returns an error

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at C:\xampp2\htdocs\index.php:1) in C:\xampp2\htdocs\index.php  on line 3

and now take a look at the moment, i can't anderstand anyway when i copy the all content of my file into another file, it start working. can somebody explain how can it happen. thanks

update

i havent't any white spaces, at least i can't see them