can i be hacked ????

Hi there
If I am right , when I connect to the net , some of my ports are open . Is it possible for anyone , to gain access to these ports and then play with my comp , without my notification . ? If yes , then can anyone tell, how they did it ? orhow can they do it ?.
Till now , I have not run any attachments of any kind . Plz tell me all the ways they can do it . If possible tell me the tools they would use and also any measures ?
Thanz a lot
Vir