Stan,

You can log in from different PCs, however, it is best to log out on one and log in from another.  This prevents an error message you might get if the IPs are different.

It's also very important that you do  not share your login with another person.  This is spelled out in the user agreement.  This is important for forum security and for your protection.  Also another reason to log out before leaving the computer you are using is so that your login remains secure.

If someone else has your login, it would be a good idea to change your password.  

There is no reason to use another's login.  Anyone who wants to log in can just sign up.  Afer all, it's free.