Windows Messenger (not MSN Messenger) for Windows XP supports SIP.
The information below is likely obsolete.
It supports Instant messaging, video and audio (telephony).
Valuable links:
Windows messenger is part of the Microsoft Windows RTC (Real-Time Communications) framework and connect to Microsoft Live communications Server
Limitations:
- Windows Messenger 5.0 did not provide DTMF entry (needed to interact with Voice Mail systems). Do not know if this is fixed in v5.1.