You should be aware that doing a port scan, while
harmless in itself, is often considered an
aggressive act by
network administrators.
There is some debate as to whether a port scan can be considered a form of attack, as it does not involve any actual breaking in. It is however often used for preparing an attack, and therefore considered an aggression, much like someone trying all the doorknobs in an appartment building to see which appartments are locked.
So only try to port scan computers where you know the administrator does not mind (or if you're sure you will not be found out).