Not usealbe unless Multisite ?

I just tried installing Signup Code on a single WP site.
It broke both the front end and admin
until I ftp ed in to delete it.

I want something like this for a single WP site that allows registration for using the Forum.

A simple captcha form for the main WP registration page would do -
but I am having trouble finding even this.

All the plugins I find by searching are either andtisplog
or else comment form captures.

Can you recommend a main registration form Captcha plugin to stop spammers ?

Thanks - DV