For a windows based SIP phone, I found that you can't really go past X-lite.
It has a download link for windows and mac os x.
Whilst I've only used the windows version with Asterisk, it works a treat. It sits nicely in the system tray and pops up when there is an incomming call. You can also set it up so the ringing sound comes though on a different sound card (ie, if you have speakers connected to one and head phones to the other, you can hear the phone ring through the speakers, but carry out the call through the head phones). Best of all it is free!