You should go to the General Display Settings page in the CP. Make sure the "Board Character Set" option is set to the standard english charset: ISO-8859-1
Then go to the Style Templates page in the CP. If you just recently installed you should have two templates, Standard and MyUBB. I'll assume you're currently using MyUBB for all your forums. Choose MyUBB and submit. Scroll down the page until you find the Character Set option. Make sure it's set to ISO-8859-1 and then submit the page. Clear your cache (not sure if this'll matter) and then your board should be working.
