Installing Captcha

Hi There,

Been trying to install Captcha to my site, as been getting a lot of spam posts, etc.

I keep getting this message when I try to enable Captcha from admin/build/modules/confirm:

Fatal error: Call to undefined function drupal_install_schema() in /home/admin/public_html/sites/all/modules/captcha-6.x-2.1/captcha/captcha.install on line 105

Can anyone help me out? I installed the captcha folder to what I thought was sites/all/modules on my server.
Sorry, I am a bit technically illiterate and new to all this. Any help is most welcomed.

Thanks!