if (isset($_COOKIE['login'])).

<?php
if    (!isset($myrow['avatar']) or $myrow['avatar']=='') {

print    <<<HERE
<form    action="testreg.php" method="post">
<p>
<label>Ваш логин:<br></label>
<input    name="login" type="text" size="15"    maxlength="15"
HERE;

if (isset($_COOKIE['login']))
{
echo    ' value="'.$_COOKIE['login'].'">';
}
print <<<HERE
</p>
<p>

print <<<HERE
бла бла бла
HERE;

print <<<HERE