If you are hearing yourself echoed then there is nothing that can be done as the echo is occuring at the other end and it is not possible to echo cancel (at your end) audio that is delay by 100ms typically.
If other people are complaining they hear echo when speaking to you, then I would advise getting a decent echo-cancelled handset. These is physically designed to reject echo making the job of any software much easier. Even local echo (which should have only a couple of ms latency) is still hard to eliminate.
Also, never use your pc speakers and microphone. Even headphones and a poor quality or non-directional microphone may pic-up echo.
As for a softphone with echo cancellation. I don't know of any; if a sound device is made for telephony it should have some echo-cancellation built in.