Trouble with HTTP headers

[hoax_monkey]

I keep getting the error message when trying to use HTTP headers:
Warning: Cannot modify header information - headers already sent by (output started at C:\Program Files\Apache Group\Apache\htdocs\scripts_named\03\register.php:49) in C:\Program Files\Apache Group\Apache\htdocs\scripts_named\03\register.php on line 51

Can anyone tell me what the problem is?
Thanks

[weevil]

You can't echo anything to the screen before using a header redirect.
Are you using printing something out on line 49?

[hoax_monkey]

This is the code I am using before the header command:
if ($name && $email && $username && $password) { // If everything's okay.
// Register the user.

	// Send an email.
	$body = "Thank you for registering with our site!\nYour username is '{$_POST['username']}' and your password is '{$_POST['password1']}'.\n\nSincerely,\nUs";
	mail ($_POST['email'], 'Thank you for registering!', $body, 'From: [email][e-mail][/email]');

    header ('Location: thankyou.php');
	exit();		
} else {
	$message .= '<p>Please try again.</p>';

[weevil]

What do you have on line 49?

[hoax_monkey]

mail ($_POST['email'], 'Thank you for registering!', $body, 'From: [e-mail]');

Thanks for your help