Any system that shipped with XP, Vista or 7:  the label with the product key is on the bottom of the system or inside the battery compartment.

For Windows 8, no key is needed - it's electronically embedded into the mainboard.

how do i do to get Windows key from main board, could you help me please

See here:

http://dellwindowsreinstallationguide.com/a-clean-install-of-windows/microsoft-product-activation/obtaining-your-oem-bios-embedded-slp-product-key-using-rw-everything-oem-only/