On what system did you buy and build the DVD?
32bit or 64bit?
I purchased from official microsoft website, windows 8.1 pro and it came as a download so i made my usb device bootable and installed on my new computer build via usb. However i checked on specs it said x86 based system which is 32bit; nowhere on microsoft website or during purchase said it was 32bit....
I just ran into this same issue.
Windows 7 32 bit upgaded to windows 8.1 pro over the MS store. Windows auto detects what your a currently running and will upgrade to that bit/version.
-- Window 32 bit can only be upgraded to a 32 version. Not windows 64! The two version are different but run the same. Easy way to put it.
Order a back up CD of Windows 8.1 from MS and do a complete reinstall to the 64 version using the same key code you got online. Cost about $15 for the Cd.
As of 2-5-14, MS has no back up CDs or windows 8.1 pro to order.
So I reinstalled windows 7 64 from a CD ( I owned) and then upgrade to 8.1 pro over the MS and now it is 64 bit.
Make a back-up of any important files because you might lose them going to 64 bit.