Cannot contect to the internet Error 720 721

Hello,
the other day after spending some time online the previous day my pc say it cannot conect to the network provider error message 720 so i tryed to make a new conection didnt work. i tryed to install a new modem thinking that it could be the modem that is the problem and then i got a message saying that a componet on my pc what not working properly so i restart my pc and then got error message 721. before the second modem was installed my dad did a virous scan and found that there was a trojan on the pc and it was removed. if any on could give me a hand it would be great