Find out DNS server IP under Windows XP/NT/2003
Click on Start button > Run > and Type command cmd > Press enter key
At DOS prompt type the command:
C:\>ipconfig /all
It will display DNS server IP address, and other information related to Windows networking:

OR
- You can Click on Start button > Settings > Network connections
- Double click on Local Area Connection
- Click on Properties button
- Select Internet Protocol (TCP/IP)
- Click on Properties button and Look for Preferred DNS Server: