After the Opportunity is set to Closed Won we would like only the Administrator and one other Key person to be able to update the record. Can I do this with a Workflow rule or would I have to create a trigger? Thanks for your help.
What do you mean by "reassign permissions?" You can simply use a validation rule to prevent saving changes on a record based on permissions, no need for a workflow or trigger. What do the Administrator and the other key user have in common? Do they both have the same profile?
AND( ISPICKVAL(StageName, 'Closed Won'), NOT($Profile.Id = '*insert Profile ID of the admin users here*') )
This will prevent anyone with that profile from being able to save changes to records once the Opportunity is Closed Won.
Alternatively, if they don't have the same profile, you can do it by User ID:
AND( ISPICKVAL(StageName, 'Closed Won'), NOT( OR( $User.Id = '*insert User ID here*', $User.Id = '*insert second User ID here*' ) ) )