Regardless of what softphone you are using, things will always work a lot better if you use a headset so that the audio cannot feed back into the mic.
Skype has invested million(s) in developing its own proprietary platform including one of the best echo cancellation mechanisms, but it comes with a price too, being locked to their service.
Since the Express Talk algorithm is probably not as sophisticated, the best I can do is suggest that either
A) You get a headset (best option)
You use headphones in combination with the built-in mic (a good alternative)
The echo problem is usually the result of the audio from external speakers being sent back through the mic creating a large feedback loop. Because of the natural latency/delay on VoIP this is perceived as a large echo as far as I understand it.
Learn more about causes of echo on VoIP here: http://www.voip-info.org/wiki/view/Causes+of+Echo