Are you using the latest version of mojoPortal? There was a bug in the global error handler in the previous version.
One thing that can cause customErrors= off to not work is if there is an error parsing the web.config file itself. It may also be possible that a setting in machine.config is not allowing you to turn this off.
There is full documentation about IIS setup here:
http://www.mojoportal.com/iiswebsetup.aspx
Note the part in the bottom half of the page has full details about the permissions needed on the Data folder and all its child objects. On Win 2003 Server its the IIS_WPG user account that needs this permission
Hope it helps,
Joe