This happened on my machine too. Microsoft seems to have kept loopholes in the Windows to force its users to do anything they want. Whenever microsoft please too they can install anyting on the users computer.
This action of forced upgrade to 8.1 is unacceptable and Microsoft has to pay for it. Like others I was also in the middle of critical work when microsoft forced a timer and started installing 8.1. Within the 13 mins timeout I tried may things to stop it, editing reg keys, trying shutdown /a etc. and none worked. I even tried forced reset to kill the upgrade but couldn't. This act of hijacking the computer needs to be explained and Microsoft needs to answer what information it is hijacking through its windows update loophole.