Open up command prompt. Press "START" and "RUN" type "cmd" and use the command "netstat -a"
Now you will get up a list so showing you poeple so are connected to your computer. This list will look like this.
Local Address: thiscomputer3123:1031
Foreign Address: thiscomputerhaha342:ftp
"Proto" showes what Protocol it is in this case it is (Transmission Control Protocol) TCP.
"Local Address" This is your computer also you see a number behind your computer name. 1031 this is the port so is used by your computer.
"Foreign Address" This is the Remote computer and the port is ftp. (File Transfer Protocol) so the port is 21 since the Default port to FTP is 21.
"State" This showes you if the computer is connected to you or not. ESTABLISHED means that it is connected to you.
So now you know how to read the result. Lets go to Step 2.
0 - 1023 = This ports are used by "Services"
1024 - 49151 = This ports are used by "Network" ect Your Internet Browser and E-Mail clients.
49152 - 65535 = This ports are used by private users they are rarly used so this may indicate that you are infected with something.
You can easy check out the ports just with do a google search. Open up your "Internet Browser" type "www.google.com" in the address bar and type in ect "Port 1031" now it will give you sites about the "1031 port" or you may search for Trojan Horse port list or something like that to check out if the port is a Trojan Horse.