Basically, if you can connect through his network, then you can access his PC. If you go through My Network Places, you should be able to go through his shared files. You won't be able to access his entire hard drive unless he's silly enough to share it.

Now, there are software applications that can monitor data transfer, and the number of users that are connected to your network.

My neighbour did the right thing and came round and told me his daughter's pc connected to mine by accident I said he can leave it if he want's to as she only surfs the net and won't take up much bandwith.

But blatantly hammering someone else's internet connection takes the right piss IMO. Fair enough he should have it secured, but he may not know how to do that.

And he pay's extra for a faster connection

How would you feel, if someone was raping your internet connection ?