The SpellChecker plugin for HTMLArea is not php but instead perl and executed as a cgi script. If you do not like cgi scripts or perl, please do not proceed.
Modify your http.conf and add or modify the following line.
AddHandler cgi-script .cgiAdd the following .htaccess file to the Spellchecker plugin directory. This directory is usually in the path misc/htmlarea/plugins/SpellChecker/ which is relative to the root of the drupal installation.
Options +execCGI
AddHandler cgi-script cgi
Order deny,allowAll going well this should then work.