How would you troubleshoot and resolve a network connectivity issue for a user who is unable to connect to the Internet on their desktop computer?

To troubleshoot and resolve a network connectivity issue for a user unable to connect to the internet on their desktop computer, an IT Support Engineer can follow these steps:

1. **Check Physical Connections:** Ensure that all cables are properly connected to the computer, modem, and router. Make sure the Ethernet cable is securely plugged in.

2. **Restart Networking Devices:** Power cycle the modem and router by unplugging them, waiting for a few seconds, and then plugging them back in. Restart the computer as well.

3. **Check Network Settings:** Verify the network settings on the computer, such as IP address, subnet mask, gateway, and DNS servers. Ensure they are configured correctly or set to obtain automatically via DHCP.

4. **Ping Network Devices:** Use the Command Prompt to ping the router's IP address to check if there is a connection to the local network. If successful, try pinging external websites like Google.com to test internet connectivity.

5. **Disable/Enable Network Adapter:** Disable and re-enable the network adapter in the computer's network settings to refresh the connection.

6. **Update Network Drivers:** Check for updated network drivers for the network adapter in Device Manager and install them if necessary.

7. **Temporarily Disable Firewall/Antivirus:** Sometimes, security software can block network connections. Temporarily disable the firewall or antivirus software to see if it is causing the issue.

8. **Check Proxy Settings:** Ensure that proxy settings are configured correctly or disabled if not needed.

9. **Perform System Restore:** If the issue started recently, consider performing a system restore to a point where the internet connection was working fine.

10. **Consult with Network Administrator:** If the issue persists and all basic troubleshooting steps have been exhausted, escalate the problem to the network administrator for further diagnosis and resolution.

By following these steps, an IT Support Engineer can efficiently troubleshoot and resolve a network connectivity issue for a user experiencing internet connectivity problems on their desktop computer.