Trademarks only apply in the domain the term is used in. So we can trademark common words already used elsewhere as long as they're not already associated with something in that domain. If I made harddrives called Banana, I could trademark Banana in that domain, but I would have no claim over it in, say, the food industry. Nor could I get a trademark on "Harddrive" in the harddrive industry.

The obvious example is Windows.