I use BigTalker, http://www.bigtalker.com
Ultraboard is another one, although I heard that it's riddled with bugs and it doesn't look too user friendly.
UBB costs $170 lifetime or $90 per year (so that's out for you). There are a few others but I would go with bigtalker. They are currently in version 4.06 beta and the non-beta release will be soon. It's free by the way.
THIS ONE. Use "Ultimate Bulletin Board" by InfoPop (formerly Madrona Park). They have a more slim free version but it is still as awesome as this! The free version allows up to 7 forum topics and unlimited posts within a forum I believe. All the administration features and registration are still intact. It's great and you know it by having posted here!
Simply create a new member (i.e. Saltire) then log on as administrator and the password as specified in the readme, then go to Member Control > then the first letter of the username, then select "Edit" of the user who you want to set as administrator. Look at the top. There is a drop down box. Set this to admin. After that, go back to member control and turn off the posting rights to the old "Administrator" member. Hope I've helped.