Sure you can and should. Any open port provides a hacker with the knowledge that your computer exists. And they can focus more attention to hacking in to it.
You should try a personal firewall software program. There are several you can get from free to $$.
I have used ZoneAlarm, Norton Personal Firewall, Sygate Personal Firewall, and BlackIce. They all work just depends on your style and needs.
for freeware and trial software