Do you have a strong firewall protection?
Open ports are like open doors thru your firewall. Exploitive programs (such as viruses) can take advantage of those open doors.
Sometimes specific apps require specific ports to be open. I don't know if you have apps like that.
Your antivirus software should have given you more details. Are you sure the warning is actually from your AV software and not a bogus ad made to look like a legitimate warning?
If you reboot, do you get the same warning?