I thank you for your prompt reply!
I have set in the user manager to use captcha recaptcha which works OK see here http://mbi.gr/en/eggrafi.html
. Also at the VM configuration / Shop tab I have checked the Use ReCaptcha for Registration but still the captcha doesn't show on the one page checkout when I check on the Register check box. I know that this is not the best way to check out but I get lots of users registering and I need to stop this somehow, that is why I have enabled the use captcha recaptcha on the new user registration.
Any solution to this? How can I show the captcha recaptcha on the one page checkout form when the Register checkbox is checked so that it goes through?