Not sure since im still running on 56k but it seems to be that the busiest times coincide with the slowest speeds on cable internet which is usually in the morning before people go to work and after they get home from work generally around 3 or 4. As to other users accessing the contents of your system I believe that they could only do that if you have enabled file/print sharing. Someone else will have to confirm my thoughts though.

PeacE
-BoB