I think you are talking about a Rain Beetle Steveb. I don't know their scientific name but they are usually known as such. If it is what I think, then the beetle is about an inch or an inch and a half long and jet black. They walk quite high on their legs and you can see under their bodies when they do.
Rain Beetles are perfectly harmless and the usual reason they come inside is that they are thirsty and are looking for water. If you place a drop of water in front of one then the beetle will often rush over and spend the next minute or so drinking. Then just pick him up gently and put him out.
Cockroaches are usually large and they have antennae about the same length as their body. Including antennae they can be about three or four inches long! Cockroaches also have an instinctive aversion to light. If you can see the insect walking around then it is almost certainly not a cockroach. They instantly scarper when the light goes on.