What operating system? (guess = Windows)
Are you setting the address? (guess = yes)
Are you authorized to attach to this network? (guess = I won't yet)
Have you tried DHCP? (Guess = no)

An IP address is a unique address within a network used to route packets. If you have two computers on the same network with the same address it's like the mailman trying to determine which house to deliver the mail to by using _only_ the address when two houses have the same address on different sides of town.

In order to prevent conflicts the system makes a broadcast with it's address when it starts up. If a machine with the same address detects this broadcast it throws up an error message on it's own screen, (so if you are not authorized on the network they know you are there..... but more fun for them they can prove it was your computer if they catch you 'cos your computer gave them a different identifier that is unique to your computer so have fun if you are doing illegal stuff), and it send a message to your machine which then disables the NIC and throws up the error on your machine too.