Try this:
Open your theme's theme.php file and find in function themeheader:
Code:
if ($username == "Anonymous") {
$theuser = "<a href=\"modules.php?name=Your_Account&op=new_user\">Create an account";
} else {
$theuser = "Welcome $username!";
}
Change to:
Code:
if ($username == "Anonymous") {
$theuser = "<a href=\"modules.php?name=Your_Account&op=new_user\">Create an account";
} else {
$sql4 = "SELECT name FROM ".$prefix."_users WHERE username='$username'";
$result4 = $db->sql_query($sql4);
$row4 = $db->sql_fetchrow($result4);
$r_name = $row4[name];
$theuser = "Welcome $r_name!";
}