If you're specifically looking for changes to the users account itself,
there's an 'Audit Log' menu at the bottom of the Setup screens that has
that information easily filterable. You can say (for example) Log Type:
User, then filter to a specific user or specific Action Type (Change,
Add, Deactivate) etc. Anthony's report is also an option that has more
information available, so it depends on your specific need at that time.
Katherine