HOW TO CHECK WHICH PORT IS OPEN AND WHICH PROCESS OPEN'S IT

Two computers are communicate with each others with the help of logical ports. There are 250 well known ports. The registered ports are those from 1024 to 49151 and IANA maintains the official list. The dynamic or private ports are those which can be used by any virus or any other protocol are from 49152 to 65535.
Here are list of some common used ports.

20 & 21: File Transfer Protocol (FTP)
22: Secure Shell (SSH)
23: Telnet remote login service
25: Simple Mail Transfer Protocol (SMTP)
53: Domain Name System (DNS) service
80: Hypertext Transfer Protocol (HTTP) used in the World Wide Web
110: Post Office Protocol (POP3)
119: Network News Transfer Protocol (NNTP)
143: Internet Message Access Protocol (IMAP)
161: Simple Network Management Protocol (SNMP)
194: Internet Relay Chat (IRC)
443: HTTP Secure (HTTPS)
465: SMTP Secure (SMTPS)

To get know which port of your computer is open, then do the following.

Manual Method

Open command promt and give the command as it is:
netstat -abn
a for all ports, b for to show which process used which port, and n to show foreign address in IP address's.

1 : Shows local ip and port bound to.
2 : Shows process associated with.
3 : Shows foreign address and port bound with.
4 : Shows the status of connection whether it is listening or established or syn-sent.

Port scanner

You can use port scanner to scan your own computer or some one else also; to see open ports.
DOWNLOAD the Advanced port scanner from HERE or Google it for free one.
Install it own your computer.
Open it and put your IP address in it. Unchecked range check box.