What does this ip address conflict error mean?

A message occasionally appears on my taskbar saying 'You're IP address is conflicting with another user on the network.'

Does this mean someone is hijacking my internet connection without me knowing?

It depends on how your network is configured.

If you have all of the computers on the network to dynamically receive their IP address from the router, then it shouldn't be a problem. However, with some windows boxes, if you have it set for them to select their own IP, it will try each IP address until it finds one thats open.

For most routers, you can assign the IP that each box on your network uses. You can also limit the number of open addresses to only what you assign. Try setting each box with a specific IP address, then keep an eye on the router logs to see if another IP is showing up. Then you'll know if you have someone 'sharing' your internet connection.

If you do have someone leaching from you, you can identify the MAC address of the NIC card they're using and ban it from your network. Then I'd go through and reset all passwords, network name and try to find out where they got in and close that door.

If your router has WI-FI, I'd bet that is how they got in. Use a WEP key on any Wireless devices to help harden your network.

Good Luck