login button

yes , you can change it.

Just go to user.module and then there is one function.

function user_login_block() {
$form = array(
'#action' => url($_GET['q'], drupal_get_destination()),
'#id' => 'user-login-form',
'#base' => 'user_login',
);
$form['name'] = array('#type' => 'textfield',
'#title' => t('Username'),
'#maxlength' => USERNAME_MAX_LENGTH,
'#size' => 15,
'#required' => TRUE,
);
$form['pass'] = array('#type' => 'password',
'#title' => t('Password'),
'#maxlength' => 60,
'#size' => 15,
'#required' => TRUE,
);
$form['submit'] = array('#type' => 'submit',
'#value' => t('Log in'),
);
$items = array();
if (variable_get('user_register', 1)) {
$items[] = l(t('Create new account'), 'user/register', array('title' => t('Create a new user account.')));
}
$items[] = l(t('Request new password'), 'user/password', array('title' => t('Request new password via e-mail.')));
$form['links'] = array('#value' => theme('item_list', $items));
return $form;
}

here change the value of submit button.
$form['submit'] = array('#type' => 'submit',
'#value' => t('Log in'),
);