There is malware that can cause this as well as machines that never saw a good cleaning so 2 links for you to check out.
1. http://forums.cnet.com/7726-6122_102-5509131.html?tag=posts;msg5509131
2. http://forums.cnet.com/7723-6122_102-569818/getting-ready-to/?messageId=5345121#message5345121
Bob
This morning I went online and did my usual browsing. After about an hour, the connection just died out. I figured it was the cable connection having problems so I forgot about it and went out for a couple hours.
When I came back, it still wasn't back on. I checked my wifi signal from my phone and that was working just fine. So, I figured I needed to restart. I did, and it worked on my PC again.
About another hour later, it happened again. I restarted it, it fixed it and went about my business. Then it happened a THIRD time. I decided to try using the Network and Sharing Center's troubleshooter. It determined the problem as "Network has no valid IP connection." So, I restarted once again and here I am.
I've tried unplugging the modem and the router and I tried re-plugging the ethernet cable as well. I'm not sure what the problem is here and how exactly I would get any help on this.
Using Windows 7 Ultimate SP1.
Modem is a Motorola SB6121 SURFboard DOCSIS 3.0 Cable Modem.
Router is a Medialink Wireless N Router - 802.11n - 150 Mbps - 2.4 Ghz.
Network adapter is an Atheros AR8151 PCI-E Gigabit Ethernet Controller (NDIS 6.20)

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