If a word is registered as trademark, can I still use it as my brand name?

You need an intellectual property lawyer to advise you on this. Some trademarks don’t conflict, some do.

Is the ad agency operating ina geographic area where you wish to Operate? Then conflict.

Are you planning to do any similar work? Then conflict.

Are you ina completely different industry? Then okay.

A good example: Apple Computers reached an agreement with Apple Records (created and owned by the Beatles) because of trademark infringement. The computer company agreed to stay out of the music business, and Apple Records would stay out of the computer business.

Over the years there have been repeated settlements as Apple Computers has developed music software and applications.

Check out the history on Wikipedia.

Yes, as long as you are not in the same business as the trademark owner.

Example (built on @zenvelo ‘s reponse):

If you’re going to sell music, you can’t use the word Apple.

If you’re going to open up a towing and car repair services, you can call it Apple.

I wouldn’t say it’s quite that cut-and-dry. For example: I doubt you could start any kind of business or brand and name it Coca-Cola, no matter the industry you’re in.

On the other hand I give you Metallica, the band and Metallica, the mining company .

Best to speak with an IP lawyer.

I just googled Metallica Minerals and they’re Canadian, which is probably how they get away with it, @Darth_Algar.

Doubt it, considering that the band has pretty much always done business in Canada as well. Then there’s the Berne Convention, which basically stipulates that copyrights/trademarks in one country are valid in all signatory nations.