have ever looked here?
http://codex.wordpress.org/Plugins/Spam_Tools#Captcha
hope that solves your problem…
I had the same problem. I just deleted the .htaccess file and it was made new by updating the ‘permalink structure’. The only thing that took some time was to recognize, that it wasn’t the .htaccess in the weblog-directory. It was the one in the parent directory!?!