It's ok and very reasonable to quit and start a business in a similar space. If you worked for 10 years in the advertising market, I expect you to start a business in advertising, not switch to hardware engineering :-)
There are some pretty clear unethical actions, that you should stay far, far away from. Basically taking any inside knowledge about existing contracts, going to customers to move them over to your new business, taking technology or code that belongs to your previous employer, etc.
I think yes, as long as you do the best you can not to use your former company's intellectual property in your products/services. I'd bet that most startups are created by employees who can provide similar products/services at higher quality and lower cost than their employers.