Firstly click start then run (search Box in vista) type cmd
You will have a black window (command Prompt) come up
Type - ipconfig /all.
you will have all kinds of info come up but the one your after is ipaddress (ipv4 address in Vista).
Also default gateway is worth knowing this is the ip address of your router, handy to know if you need to configure it i.e firewall settings, connection settings ECT