It depends.
If a name becomes well known, like Walt Disney's name is, the name has a "secondary meaning". It becomes off-limits for any use that may confuse a consumer, whether the name is registered or not. Walt Disney, Howard Johnson, Ralph Lauren, McDonald's, and Ben & Jerry all all names that have become synonymous with a product or service. So, even if my name was Ralph Lauren and I was a clothing designer in my own right, I could not use my own name to market my clothes.