like has been said, legally it is a no no, pretty sure it says in the EULA that you can only install 1 copy on 1 machine. I think it would probably work though, the serial number is there for online accounts, not sp or LAN, atleast I don't think it applies to LAN
Doing the LAN itself isn't an issue, the installing the same copy onto several machines is the bad bit (I think?? Correct me if I am wrong please someone )