CC version 3.0.13 - I have added an HTML link to a blank target page detailing Terms and Conditions on the bottom of the Registration page. Could this be the cause of the following error when attempting to register:
Fatal error: Call to undefined function: addslashes_gpc() in /home/nextgen/public_html/includes/content/reg.inc.php on line 99
Any help greatly appreciated.

I doubt it was the link that did that, seeing as you would have added the link to the .tpl file and not the .inc.php file.

Do you get the error as soon as you click the 'register' link that takes you to the form, or when you submit the form?

I'm looking at the reg.inc.php file in version 3.0.14 and there's no reference to that function anywhere near line 99 as far as I can see.

Download the latest version and compare the two files, maybe you'll spot something.

Let me know how you get on