get-ciminstance win32_bios | format-list serialnumber
Here is what the above code is Doing:
1. Get-CimInstance is a cmdlet that allows you to query WMI.
2. win32_bios is the WMI class that contains the BIOS information.
3. Format-List is a cmdlet that allows you to format the output of the command.
4. serialnumber is the property of the win32_bios class that contains the serial number.