Find out the IP address of the desired computer. You can do this, for example, using an email program MS Outlook. Fulfilling the letter from the right person, right-click the mouse on the address of the correspondent. Select "Properties" from the menu and navigate to the tab "Details". String Received: from contains the sender's name and IP address. In the case of sending emails from a computer connected to the local network in the row will contain the network address of the gateway.
Use one of the Internet resources that allows you to locate your computer and get information about it via the IP, for example, "2ip". These sites provide the necessary data based on Internet service provider which caters to your computer. Click on the link "Information about the IP address..." and select the appropriate combination of numbers. In the results you will see the geographical position, legal address, phone and other available data.
Use another popular resource, the "IP Whois". Click on the link "information about the IP" in the right side of the main page. Click the address in the corresponding line and press "Run". In addition to the geographical location of the provider you will be able to see the estimated location of your computer on Google map.
On that computer, install free app LanWhoIs that collects information about domains and IP addresses. You can download it on the website of the developer. Enter the desired combination of numbers in the "Address" field and hit "Query". To save the results, use the Save command, which is available in menu "File".
Remember that some users take special measures to protect the computer and hide your IP address using proxy servers or anonymizers. In this case, you will not be able to establish their whereabouts.