For MB like Intel 9X series, you could see the barcode near the ATX Power connector (JPWR1)
For MB like Intel 8X series/AMD MB series, you could see the barcode Sticker at the PCB side of the motherboard
System Information (also known as msinfo32.exe) shows details about your computer's hardware configuration, computer components, and software, including drivers.
Note: You can save System Information by clicking File, clicking Save, typing a name for the file, and then clicking Save again. Windows saves this information in the NFO file format. If you have trouble opening this file, you can save System Information in the TXT file format by clicking File, clicking Export, typing a name for the file, and then clicking Save.
Please run MSI Help Desk software and click System Info at the left hand column. The software will list your system information at the right hand column including serial number.