Add javascript button to VB registration page

[CampinCarl]

Thanks, I actually knew about the custom user fields but didn't think to work my javascript around them.

Now one last question until I have this small project completed (and I did look in the manual but couldn't find help on it!):

When a user is registering, is it possible to check a custom field to see if another user already has the value in that field? I don't want any 2 users having the same value in the custom field I created.