[grabeh]

You can trade mark any word if it is sufficiently distinctive and can uniquely identify you as the provider of that product and/or services. For example 'Computer' for canned goods would be fine, but obviously registering 'Computer' for computers themselves would not be distinctive.

You can also only enforce your mark against someone using a mark in the course of business (at least in the UK) which would obviously exclude a whole range of potential third party uses.