Use check_markup() to filter your output in drupal 5

check_markup($text, $format = FILTER_FORMAT_DEFAULT, $check = TRUE)
 
//use it like this in for example a node template
$body = check_markup($node->content["body"]["#value"], $node->format, FALSE);
$field_whatever = check_markup($node->field_whatever[0]['value"], $node->format, FALSE);