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This would make subdomain.domain.com/forum an impossible location for the phpbb message board that comes with cpanel, wouldn't it?
Nope, installing it on a subdomain works
just select your subdomain.maindomain.com domain from the drop down menu and add /yourforumname to the forum path (folder where you want to install the forum)
Anyway, if you want to do it otherwise here's how to chance the script path:
I'm unsure about Invisionboard as I'm mainly used to phpBB2 (plus even) but here's how to do it in phpBB2:
1) install phpBB2
2) Go to your forum and login as administrator
3) Click on the 'Go to Administration Panel' (all the way in the bottom)
4) Click on 'Configuration' from the Admin menu on the left
5) 3rd option from the top is named SCRIPT PATH and it will say something like /phpBB2/ (which is the folder where you installed the forum)
6) Set the new path you are about to use (which will cause the forum to fail and go offline, but as soon as you have moved it to the new path it will work again):
Ex: If you install phpBB2 on subdomain.maindomain.com/phpBB2/
your scriptpath will be /phpBB2/
If you now wish to move it to subdomain.maindomain.com/subfolder1/subfolder2/phpBB2/
just set the scriptpath to /subfolder1/subfolder2/phpBB2/
That's it
Note: Invisionboard is more advanced and faster than phpBB2 as it is optimised especially to work with MySQL databases while phpBB2 is easier to tweak and support miscellaneous SQL formats. Other forums are not recommended as they're crap
Hope I enlightened you
Regards
Toph1980