If you are looking to exlude feedapi nodes or webform nodes or maybe even node-types you ve created yourself using cck from your search results because they are not relevant.
Use http://drupal.org/project/search_block [4]
Install and go to the settings page(settings > search > tab)
You ll be able to restrict nodes on a content type base from your search.
If your site was allready indexed you should re-index it. Go to settings > search to do it. Run your cron until everything is indexed.
Links
[1] https://dominiquedecooman.com/blog-topics/drupal6
[2] https://dominiquedecooman.com/blog-topics/drupal5
[3] https://dominiquedecooman.com/blog-topics/search
[4] http://drupal.org/project/search_block