is there another way of securing your register page from robots, I'm thinking about doing my registration page as a flash (swf, my site requires flash for video streaming anyway), would that help, or not? what else can I do?
A lot of people (like me) use Flashblock to remove flash from pages. I'm not saying that all flash is bad, and I make exceptions for sites that I know and trust, or that are well-known and rely on it (e.g. YouTube), but I don't allow flash on some random page. It is generally just an ad or other distraction, and could be a flash cookie or even worse.
If your goal is to keep from having to implement this yourself, you may want to take a look at http://recaptcha.net/. They have a free web service that generates captchas and by using it, your users will also be helping to digitize books.
... and then your SWF flash page will post a HTTP request. Which any minimally skilled spammer can capture, and automate the HTTP POST to register on your website, instead of interacting with your flash content.
CAPTCHA means "Completely Automated Public Turing test to tell Computers and Humans Apart". A flash interface only raises the bar, but won't address the issue at hand. If you are really interested in trying other methods, check these turing test alternatives from W3.org
On the traditional CAPTCHAs, you should check reCAPTCHA before ditching that approach. I use them in a few projects and found it easy to integrate and does the job well.
Good luck there, human! :)