Check applications listening ports

Run as administrator:

netstat -a -b

From the output we can see that filezilla is using port 21:

Proto  Local Address          Foreign Address        State
TCP    0.0.0.0:21             item-ax32034:0         LISTENING
[FileZilla Server.exe]
TCP    0.0.0.0:135            item-ax32034:0         LISTENING
RpcSs
[svchost.exe]
TCP    0.0.0.0:443            item-ax32034:0         LISTENING
[vmware-hostd.exe]
TCP    0.0.0.0:445            item-ax32034:0         LISTENING
Can not obtain ownership information
TCP    0.0.0.0:902            item-ax32034:0         LISTENING
[vmware-authd.exe]
TCP    0.0.0.0:912            item-ax32034:0         LISTENING

Check which application is using a certain port

From Power Shell:

netstat -a -b | Select-String -Pattern :21 -Context 1,1

Output:

Proto  Local Address          Foreign Address        State
>   TCP    0.0.0.0:21         item-ax32034:0         LISTENING
[FileZilla Server.exe]