To "trademark" something in the USA you would have to be the first to use it on particular goods or services "in commerce", as a designation of source or quality. A trademark doesn't exist "in a vacuum" -- it only applies where you have actually acquired the rights by using it.
That said, if someone has a domain name that uses a similar (or identical) "brand" to market things and your use would create a "likelihood of confusion", they could probably stop you from marketing similar things in similar channels. However, if they are simply using the domain name as the internet address identifier it is, and not actually distributing anything with it, then it is not a trademark and they would have no particular claim to prevent you from using it as a trademark in just about anything you want.
Like Hunch says, there is a REGISTERED trademark for McDonald's plumbing and cooling products that did not prevent another company from registering McDonald's as a brand for restaurant services and related products. Similarly, LEXIS and LEXUS are phonetically "identical" but have been ruled to not create "likelihood of confusion" because their respective customers are smart enough to know the difference between information services and luxury automobiles.
Bottom line, if they were there first, you have to make sure you're not infringing their rights, if you don't want legal trouble, but you can certainly be "first" in the use of it for other things.
Trademarks are based on the specific category. For example: it would be legal to have a McDonald's, the restaurant, and McDonald's, a plumbing company or something completely unrelated to restaurants.
Additionally, if the existing company is "in your space" they are actually the trademark owner and your trademark can be revoked.
Possibly….a federal trademark gives more protection than a domain name. But if the domain owner finds out you are applying for a trademark, they can file an objection (and probably win based upon prior use). Even if you get the trademark, they could probably still keep the domain name, they just couldn't market their goods/services under the same trademark (maybe).
I don't believe you can effect preexisting businesses, just guessing there is probably a clause protecting them. Otherwise its' too easy to hijack an up and coming site that couldn't afford to register a trademark at the start.
You can look up Cyber-squatting laws to learn the protections from someone crating a site after the name is trademarked.
If I trademark the word "helloworld" for example, can I do anything about someone who has a domain name called helloworld.com?
