About the ports, some ports are useful and some are not, reason:

21 FTP
22 SSH
110 POP3

This is a list of the most commonly hacked ports, some of them are only used for stair stepping into a system and some of them can be cracked with the right credentials and give you full access... For instance, SSH port is open, you connect to it using PUTTY and type in the correct user name and password and there you go, full access. FTP port is open then you will only be able to send and receive files on the target computer once logged in. HTTP is open and you telnet to that port to get very valuable info from the computer like the OS and such. TELNET port is open and you will have full control of the system once logged in with the right credentials. SMTP is only for sending emails and if the server allows for anonymous emails you will be able to send emails on behalf of someone else. POP3 is for receiving messages and is quite useless but can be used to find the type of OS. Now, all the other ports will give you a hint as to what type of server is running and what services is running with what OS. SSH is mostly Linux, so the server will be linux 90% of the time, TELNET might be a router but it can also be an OS, so connect to that port an check uot to find out. port 135 is open then it mean that its more likely to be a NT or windows box... So we can go on and on, follow this link if you are more interested: http://www.go4expert.com/showthread.php?t=8841