I need to insert a captcha into a form...anyone know how?

Recaptcha is quite nice:

http://www.recaptcha.net

Couple of others:

http://captchas.net
http://captchator.com

You can use a combination of GD2 (Imagemagik too if you like) in php to render your own.

Its worth noting that noise added in the 'captchas.net' captcha is unlikely to thwart any determined OCR after the noise has been reduced. Much better to randomize the backgrounds and distort the lettering.

See here for different captchas and their weaknesses:
http://www.ocr-research.org.ua/list.html