All your apps and the operating system are stored on the hard drive so no you will not need to reinstall everything. You will just need to reinstall the drivers for that new motherboard however as the operating system will need drivers to function components like the internet port and other various features on your new board. You can find these drivers either on the DVD that comes with your new motherboard or in digital versions on the official website of the board from the vendor (simply google the board's name and it should have a direct link to the board's page).