We need to make a distinction between key validation and OS Activation
Key validation allows the install to start. You can have your key validated while offline.
The activation is done at the end of the process after the install is complete.
For key validation issues check for the common errors ( I char looks like a 1, G looks like 6, O looks like 0) while you enter the key
For activation: if you did not install with a "used" key check your internet connection
or even update your net adapter drivers (in the off chance you are getting corruptionn)
-I have seen people plug in a phone cable into their laptops Ethernet port when they could not get their wireless access to work. (falls under bad network connection)
all else fails you should call Microsoft product support and make them figure out the problem.