How do I stop Spam User Registrations on my Site

Hello! So I have a problem with hundreds of Spam Users registering on one of my sites each day. Very frustrating and the client is getting mad. Yesterday I spoke to a WPMU tech who said I should use:

SI CAPTCHA Anti-Spam

To setup captchas on our registration page and commenting functions. Unfortunately it did not work to prevent them and we got hundreds again today.

I am aware of Stop Spammers plugin however when I tried to install it it overloaded the server and crashed the site. I also know of Register Plus Redux which might be a more sophisticaed CAPTCHA, but I just don't think a captcha will work that well (at least thats what seems to be the case after reading a number of articles).

Is there another solution/plugin/method you guys could suggest? Thanks!

AD