You’ve got a problem with your computer, you call their technical support team and the person on the other side requires the machine’s serial number (also known as the Service Tag) before he can even log your request.
dell service tag          If you using a laptop computer, you can easily find the service tag by turning the machine upside but for desktops, especially the older models, the sticker that holds
the service tag is often placed at a more harder-to-reach location.
           There’s a alternate way to find the computer’s serial number without you having to slide beneath the table.
Open the command prompt (Start – > CMD) and type the following command:

wmic bios get serialnumber
 
Another command that will also print the model number (or make) of your computer is:

wmic csproduct get name, identifyingnumber 
 
The system model number can also be found using the System Information program available under All Programs – > Accessories – > System Tools.
The above commands should work on Windows 7, XP and Windows XP Pro machine as wmic.exe didn’t ship with the previous editions of Windows. Refer to this page to learn about all other system details that you can retrieve with the help of wmic.exe command.