I've had this problem for several weeks now. I first experienced it several weeks ago, with my wireless card; I'd be connected fine for approximately 20 minutes, then the connection would drop. If I looked in 'Network Connections' at this point, my wireless adapter is not showing up. I then took the 'powerline' ethernet plug, that I had for my Sky+HD box and used it to create a network connection via. ethernet. I changed the 'channel' of my router, and the problem still occured (except the connection drop occured approx. after 40 mins each time)
Now the problem is occuring again, but this time it is the 'ethernet cable' that is not showing up!!! I'm now using the wireless connection which was originally defective in the first place. When I look in device manager the Intel 82579LM Gigabit Network Connection (Ethernet Port) is showing, but the properties list the device as 'This device cannot start. (Code 10)'.
Here's some information about my 'setup':
I am running a custom built PC
On my PC I am running Windows 7 Home Premium 64 Bit
My motherboard (with inbuilt ethernet) is a Asus P8Z77-V
My wireless card is listed in device manager as an Atheros AR9287
I use wireless, due to the nature of my house. We are unable to 'run cables through the building' due to it's contstruction
Try a usb network adapter if you can get a hold of one, which will let you know that it is your drivers/hardware if that works fine.
Yea, honestly the only solutions I can see here is making sure you have the right drivers. Boot into a linux dvd or usb and if it connects to the internet just fine, then you know it is windows interface with your hardware that is failing and not the hardware itself.