Since your machine came with Windows pre-installed, it's based on an OEM license.
OEM Licenses are invalidated by replacing hte motherboard outside warranty conditions - you will have to purchase a new Full Retail license for Windows.
When your machine goes ouside the Grace period, you will get a non-genuine notification, and the backgrund will go black. You will also lose access to certain things.
You will, however be able to coninue using the machine pretty much as normal.
Once you've bought a new license, you can simply swap the Key
into the current install (assuming you buy the same edition as currently installed.