Get the software and find out yourself. This ALSO means that it is possible for someone to execute what's called a 'port scan' on your computer. Programs that do this are small, efficient, and good at what they are designed to do - find holes in your security. Every computer on the internet has a unique address called an IP address. With cable these are naked to the world.
Someone can type in an address, and run a port scanner.
Most computers running windows have at minimum 3 ports. After someone finds an open port they can start sending questions, pokes, prods, 'fingers', and even commands to your computer. This could let someone know anything from where you live, to install secret software on your computer. Software was installed on their computers that acted as an amplifier for the attack. For every single data packet sent to their computer, the program recieved it - looked at it.