Are macbooks that hard to hack into?

I have a sony VAIO and my cousin has two MacBooks and in our internet modem the little trojan appears. I haven't seen it but he had a guy come check it out because when one person is using the internet it shows that 2 people are connected. We already changed the password and everything, but it always shows one more connection than what there is. They think it is me who has the virus but I have checked and I always check for viruses and nothing has ever shown up. I don't know what to do anymore. Besides my laptop has not been acting funny. His laptop is the one that wont allow him to go to certain website pops keep coming up and it runs slow, despite the fact that we have fast internet. It sounds to me like he is the one with the virus, but he insists that MacBooks are inpenetrable. Is that true? Is it possible to not detect a trojan even when I scan my laptop? I have Symantec Antivirus by the way.

Are macbooks that hard to hack into?panda

It sounds like you are checking the "attached devices" section of your router, which I believe lists recent DHCP leases the router has given out. Many routers will continue to show a computer here *after* it has disconnected. The computer will also ask for a dhcp lease when it is powered on, even if no-one is actually using that computer to access the internet.

A virus could *not* cause this, but you could have someone freeloading off your wireless connection if you have one (such as your neighbours). If you have wireless, remember to login and set up a WPA encryption password to stop this.