hi,
you have only to look what's in the login block.
you can add in your header something like that
<form action="http://yoursite.com/user.php" method="post">
Username:
<input type="text" name="uname" size="12" value="" maxlength="25" /><br />
Password:
<input type="password" name="pass" size="12" maxlength="32" /><br />
<input type="hidden" title="rememberme" name="rememberme" value="On" />
<br />
<input type="hidden" name="xoops_redirect" value="/" />
<input type="hidden" name="op" value="login" />
<input type="submit" value="LOGIN" />
</form>