I basically want an anti spam question that changes everytime, however every time i go looking for a script i keep ending up with image captcha scripts, is there a specific term for the question and answer scripts?
I'd also like if possible (but not essential) to see a tutorial on writing one, so i know how they work.
The best anti-spam protection comes from not being the same as every other site running the same software.
Add one text box that says "enter the 4th letter of this domain name here" and another textbox with a hidden attribute that automatically bans anything/anyone who adds anything in the box while registering. Those two combined will block most bots though it will fail if the bot is monitored by a human.
The exact code will vary by software type, you didn't mention what you're running.
you could look at a few that have been written for open-source stuff like wordpress and phpbb forums, and deconstruct them. have a look through phpbb's MOD directory, or the wordpress plugin directory. there's loads of php CAPTCHAs in there.