[X4U] Remote access to sleeping machine

[Ed Graf]

I hope someone can answer this question.

A friend of mine with a G5 iMac was concerned that, should he leave  
his sleeping computer with an always on connection to the internet,  
could someone, somehow, log onto and somehow invade his computer?

I have remotely connected to _my_ computer, but it has always been  
awake to start with.

If one can successfully log onto a sleeping computer, what can one do  
to immunize oneself from such an occurrence, short of turning off the  
machine?  I showed him the built-in firewall, but, I suspect I was  
not clear enough on what to do.

Thank you very much, in advance,

Ed