It's either the modem/router, the cable or the desktop/laptop. A laptop is easy to try elsewhere, a different cable is also easy to try. If you have a separate router (not integrated in them modem) it's easy to try another one too.

Trying a desktop somewhere else is a little bit cumbersome. It might be easier to ask a friend with a laptop to try with you. If that works well, the final test - alas - involves trying your desktop somewhere else.