Installing Bootcamp 5.1 on Windows 8.1
I just faced the same problem.. and this might be the solution. 

I use MacBook Pro late 2011 and installed Windows 8.1. 

(prepare your USB min. 4GB)


ensure you download the Windows support software from Apple Bootcamp.
you will get Windows support folder in the USB. 
restart to Windows
open your USB file there.. 
you will find setup.exe
right click on your setup.exe
go to compability tab
check the box of Run this program in compability mode for (and change the list to Windows 8)
apply
right click again on setup.exe
run as administrator
and click. run the program without getting help. 
installing
restart
and voila... everything works.