You get "There is an IP address conflict with another system on the network." or "The static IP address that was just configured is already in use on the network. Please reconfigure a different IP address" or similar message when trying to connect to network.
When computer connects to the network it tries to get an IP address for itself. If computer is using dynamic IP address allocation (DHCP protocol) then router will provide a new IP address to it. But if static IP address is defined in network settings for that computer then IP address conflict could occur.
Check if you are using static IP address
Go to Control Panel -> Network and Sharing Center
Click on Change Adapter Settings
You will see a window with connections that you have on your computer. Select the one you're currently using (either Ethernet or Wi-fi). Click right mouse button on it and select properties. In a dialog called "<your connection> properties" select "Internet Protocol Version 4 (TCP/IPv4)" in a list box named "This connection uses the following items:".
As a result you will see a dialog named "Internet Protocol Version 4 (TCP/IPv4) properties". If "Obtain IP address automatically" is selected then it means that your computer doesn't use static IP address and tries to get IP address from network automatically.
If "Use the following IP address" is selected then check what is specified in "IP address" field. IP address there is static address assigned to your computer. And if this IP address conflicts with any address on the network you will get an error message described above.
To resolve conflict make sure that all static IP addresses on the network are different and don't conflict with each other.
Renew IP address for the computer.
In cmd.exe launched under Administrative privileges run following commands.
This will force your computer to request new IP address from the network.
If none of the methods above work then it makes sense to replace router/modem to see if problem is hardware related.