* If "Dr. Williams" refers to a specific person, then yes, it's a proper noun.
* If "Dr. Williams" is used generically to refer to any doctor with the last name Williams, then it's not a proper noun.
Think of it this way:
* Proper noun: Dr. John Williams (referring to a specific individual)
* Not a proper noun: A Dr. Williams (referring to any doctor with that name)
Let me know if you have more context, and I can give you a more specific answer!