Open up command prompt, type netstat -n, and see results. Open up another command prompt, start a file transfer with the victim, and type netstat -n into the new window. Look down the bottom for one extra IP address since the first command you entered - that is their IP address.
Start -> Run -> cmd
That is how to open a command prompt, if you didn't know.
OR, there are many other ways to get IP addresses, since they are a number required for all connections between two computers. But the netstat & file transfer combination seems to be the most popular.