Some hosts have <forums> pre-installed
All the functionality of these forums is pre-installed by some hosting companies. With ours you just pop in to the Admin console, activate an option, and everything works on via the URL forums.mydomain.com.
Our hosts are based in Geneva Switzerland and charge 80 euros per year per domain (correct Sept 09) for all you could want and more. This is just one of many scripts they offer pre-installed, including chat and a basic store.
Perhaps your hosts already offer this, or perhaps you could change to one?
Otherwise, every script I need I have always found something Open Source and satisfactory. Try searching for "php open source forum".