Ethernet + Arduino

Hello forum,
I have an Arduino Ethernet Shield and I put it on an Arduino Mega 2560.
When I connect the Arduino Mega alone, it is recognized and I can upload any program on it. However when I connect the Ethernet shield on top of it and also plug the RJ45, the Serial port still is recognize that there is something plugged however gives me error to upload.
avrdude: stk500v2_command(): unknown status 0xc8
avrdude: initialization failed, rc=-1
Double check connections and try again, or use -F to override
this check.

avrdude: stk500v2_command(): unknown status 0x01
avrdude: stk500v2_disable(): failed to leave programming mode
I am using the windows 7, because on windows vista continued to show unknown device and did not even recognized that there is any serial port.
When I try to update driver, the windows say that the drivers are up to date, however still is marked as unknown device.
I also read many answers on different forums and tried changing the usb, I also changed the OP and many other things.
so any help is really appreciated.
I believe that it is a problem to download the ethernet shield driver... if so can somebody tell me from where I can download it??
thanks a lot
regards, nestafred

nestafred:
I believe that it is a problem to download the ethernet shield driver...

Where did you buy the Ethernet shield from?