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Please help! I am at my wits end. I am by no means a techy, but I feel I am intelligent enough to read these forums and follow directions, but I digress...
My issue, I have a very simple home network which consists of a DSL modem, a router, and 2 computers (1 laptop and one desktop,both running MS XP SP3 and hard wired into the router). All of a sudden my desktop computer stopped being able to share on the network and then eventually it lost its internet access as well, all the while the laptop is completely unaffected. I've literally tried EVERY fix that I found/read including changing my network adapter and updating the drivers, swapping out the cables, and changing the configuration of the network adapter. I've run through a series of pings. The working laptop does not see the desktop. The desktop fails when pinging the default gateway and the laptop, it is only successful when pinging itself. I've noticed when monitoring the attached devices from my Netgear browser interface on the laptop during a reboot of the desktop, it shows up but eventually disappears (a couple minutes after the reboot cycle completes). I've even noticed that I gain internet access after reboot, but only for about 5 min before it goes away.

Has anyone seen this type of behavior? Could it be software/firmware related? I am having real trouble believing its a hardware issue as I've replaced all of the network hardware on that side of the router related to the problematic desktop, and the rest of the network hardware seems to be fine (in my assessment) as the laptop functions perfectly. Any advice or help would be greatly appreciated as this computer being on the network is essential to my day to day work. Thanks, Wits End

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This is usually caused by a basic configuration error. If the modem is doing NAT, DHCP must be disabled in the router and the router's WAN port must be left disconnected. If the router is doing NAT, the modem must be set to bridging mode and the modem must be connected to the router's WAN port. (Assuming you have one of the two most common configurations. There are other configurations that work too, but most other configurations that you might get by accident do not work.) – David Schwartz Apr 18 '12 at 22:15
Thanks for the reply David. My Router is using DCHP and has been working in its current set-up for approx 2 years with no problems. I have limited troubleshooting experience, but it almost feels like an undetected virus/spyware is on that computer or something preventing the computer to access the internet/network. I've had McAfee AV running continuously in the background for as long as I can remember with no issues (again, i'm just spitballing here based on limited experience). – Joe Money Apr 18 '12 at 22:30
It was working. But remember, between then and now you've been doing lots of troubleshooting, swapping wires and so on. It's entirely possible that while trying to fix one problem, you created another by, for example, reconnecting your modem to the wrong port on your router. – David Schwartz Apr 18 '12 at 22:35
OK David, you could be right. I triple checked my connections, rebooted, and the same problem exists. Again, this problem occurred BEFORE any changes to my network were made. The computer basically just stopped accessing the network/internet. The weirdest part for me is when I reboot the computer, view the attached devices from my other computer, the problem computer appears and then disappears. Why would this computer show up in the list in the first place if it couldn't access the network? Is it possible its something within the computer that's the problem and not the network? – Joe Money Apr 18 '12 at 23:02
It could be a matter of who wins the ARP race. If the router always wins the race, then only an unsolicited ARP will make traffic go to the machine. You can check the ARP tables. When the computer isn't working, can it reach the router's administrative web pages? Can it ping the router? Does it have the router's IP in its ARP table? Is the entry correct? (Did you make sure your modem isn't routing too? What make/model is it?) – David Schwartz Apr 18 '12 at 23:11

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