I used to take X-lite as my VOIP softphone client on my Windows 7 machine. Personally speaking, it is just OK. Someone recommended Express Talk VOIP softphone to me couple weeks ago, and I have been using it since then. I think it is much better than X-lite in terms of UI and stability.

With Express Talk business version, you can use up to 6 VOIP phone lines. If you just want to use it at home, you can choose the free version which supports 1 line only.