mutantsushi on 22 November 2016
Windows 8.1 along with Classic Shell Start Menu extension works great.
I believe there is a "roll back" to revert to previous OS version which may still be saved on your drive somewhere.
If you had an "installer partition" on your drive, that could still have the original OS version ready to install fresh.
Did you not make installer/boot disks/USB? Even if not, the computer came with licence for 8.1.
So you should be able to contact HP and they will send you new install media or otherwise help you.
