Itl took  more 10mn for activation after saying:a problem occured when windows tried to activate.Error code 0x80072F8F.For a possible resolution,click more information or contact Microsoft by phone to help resolve the problem.When click on the code it said:description a security error occured.You have to check your source to see if everything works like the past.

A customer find this problem when activating system using our keys. This issue may occur if the date and time on the computer varies too much from the valid date and time of the Secure Socket Layer (SSL) certificates that are used by the Windows Update Web site.

Solution: reconfigure your PC time , adjust them to the correct date and time.

More other methods can be found  http://support.microsoft.com/kb/929458