The "cable unplugged" message is saying that it does not have a link with the downstream station or router. So, try plugging the cable at the router end in to one of the other switch ports in the router and see if the symptom changes. You may just have a bad switch port in the router ( had a Linksys router fail this way once... and the rest of the switch ports died off slowly as well...)
If your PC is setup to get it's IP address from the router, then if the connection is disrupted, the DNS information will also go away, along with the IP address... (from a command window type: "ipconfig/all" ... that will show you if you've lost all of the setup)
If your desktop PC has more than one wired interface (possible) then turn off the one you are not using for the wired connection. The Windows IP stack will swap between the two and cause problems. Setting a specific metric numbers can also help - but let's just keep this simple for the moment.
Now if neither of these has made a positive change then there is one other thing you can try:
Try this temporarily (and make notes of everything you change in case you need to go backwards or something changes unexpectedly)
- Assuming that your D-Link router is at 192.168.1.1 on the LAN side:
1) Find out the IP addresses of the DNS Servers for your ISP
2) Configure the D-Link router to disable the local DHCP server in the router so you can set a static IP address on your local network ( Can I assume this is a home network and not a business network ?)
3) Configure your desktop to use 192.168.1.10 and then manually set the DNS Server IP addresses in the desktop PC to the addresses of your ISP's DNS Server(s) in the Windows PC. We want to bypass what the router might be providing and go direct. This is done on the Network setup page on the PC.
4) Check the D-Link Router connection status page and make sure it has a stable network connection with the ISP. Leave that browser page up so you can refresh. The desktop PC can have a perfectly fine connection with the router, but the router may not be talking to the Internet.
This should make the problem go away. If not, post any new or changed symptoms here and we can continue troubleshooting.
Good luck, I hope this helps...