Are you using the built-in mic jack? Those are notoriously bad. Cheap USB's that give you the same set of jacks, aren't much better.
Get a good USB audio interface, that is designed for the serious pro market. (and not for audiophools - those marketers will say anything...) They're a bit more expensive of course, but not by too much. You might pay more for a good mic and its accessories than for the interface.