On some BIOSes programs like Rufus and UnetBootin, which extract the image and use their own Syslinux or grub-like boot systems, refuse to work. I have had this problem many times on some older computers.
A process that works for me a large portion of the time, (And is my perfered method) is to write the image directly to the device.
WARNING: This process will destroy your data on the drive, and wipe any installed partition tables. To restore your drive to full working order, you might have to use gParted or another partition system. (Windows sometimes does this, but if it your USB device comes back with less space then it started with, you need to use something like GParted for Linux...) I am not responsible for any data loss, nor drive corruption as a result of using this guide.
DISCLAIMER: I am not responsible for any data loss, nor drive corruption as a result of using this guide.
NOTE: You will also have to be an Administrator on Windows and have access to use sudo on Linux.
First, you need the program you want to use based on the OS you are using.
For Windows I recommend Win32 Disk Imager.
For Linux, The command line dd tool.
Instructions for Win32 Disk Imager
This process is quick and easy, once downloaded, extract the .zip folder and find Win32DiskImager.exe start it and allow it to be run as Administrator.
In the Image File Section, click the blue folder and change Files of Type to "." in the drop-down box at the bottom of the dialog. Navigate too and select the Windows8.1 ISO you are using.
Select the drive you will be writing the data to, this box is to the right of the Image File Section area, Make sure the USB drive has enough space for the ISO!
Double check that the drive letter is correct and click "Write". Wait for the progress bar to finish, close the "Complete" box and the program, then Eject the drive using the icon in the System tray.
Instructions for dd on Linux
This one is a bit more involved. First you need to know the /dev point for the USB device. The easiest way is to use the answer to this SuperUser Question. Normally it is /dev/sdb.
First you must unmount the USB device.
sudo umount </dev/ point>
You must also define the partition, for example /dev/sdb1
Once you have that, use the cd command to get the directory the ISO is stored under. You can then use dd to write the ISO to the Flash drive.
sudo dd if=<Windows ISO FULL file name> of=</dev/ of usb> bs=4M
Where you replace the various parts as necessary. Type your user password and if you can run sudo commands, the terminal will "freeze" DO NOT PANIC, this means it is working. This will take upwards of half an hour to complete.
For either of these commands, if you don't have sudo access, the terminal will warn you and possibly say you don't have access to sudo, and it has been reported.
After the command line returns and is awaiting your input again, you can unplug the device from the Linux system.
Plug your drive into the computer you want to install Windows 8.1 on, and select it to boot from. (Either by F12 on the boot splash screen or through the BIOS settings) Hope this works for you!