Peter,
I have noticed that if you change the Indexing Service Catalog it seems to change the security of catalog.wci folder in the DMX folder. Then you end up with errors in you file manager and on the settings page of all your modules.
Or this could be happening the setting up of the full-text search. Either way I have had it happen to me twice. Once I give catalog.wci Network Service full control of the folder all the errors go away.
Obviously I am running DNN on Server 2003 with SQL server 2000.
Brandon