It could be a driver issue.
Even an ethernet port needs the correct driver to work, and it may be your network adaptor is using a generic driver, after re-installation of Windows, rather than a more efficient specific driver.
Go into Device Manager and see if it's using the most up to date driver.
this would be my first thought too. very consistent with the fact that the OP has just reinstalled windows.
I would say it would be worth a visit to the PC/motherboard manufacturer website.