how do I ensure usernames are unique in f'page & store in access

[sally]

I am trying to set up a website where users need to register, not necessarily
login, but they need to be able to submit other forms which require their
username. I want to store the information from all forms in an access
database and therefore need the username registration information also stored
in the access database for data normalisation purposes and ease of data
manipulation within the database. The username field is indexed in the user
registration table and has a one to many relationship with many other tables
in the database (all of which attain their data from the website, and I need
to know which user submitted each record).

I cant find any info on how to link the frontpage wizards user registration
form to the database to direct it there, I also cant work out how to display
a more user friendly message from access than the indexation error message
which automatically comes up if the indexed field's new data is not unique.
How do I put my own message in there? Do I do this through frontpage or
access?