I have Windows 8.1 and I tried to dual boot Ubuntu 13.10 with a USB drive. The installation seemed to work fine (I installed Ubuntu on my third hard drive). Now how do I boot to Ubuntu?
If your computer came with windows 8 or 8.1 installed there will be an option in your bios settings to boot into legacy or UEFI mode. (You can access your bios settins by pressing f2 or f8 or a different key depending on ur motherboard, google it and it should be easy.) While on your windows partition, create a bootable usb stick using a software called rufus, because rufus gives you the choise to create a usb stick that can be used on UEFI or legacy boot. Chose and create your usb for MBR bios (legacy). Access your bios and change it from UEFI to legacy mode. Once you boot your computer with your usb stick plugged in, install it in a partition of your choise. The only downside to that, is that your every time that you want to boot to your other operating system you will have to change your bios settings (to UEFI if you want to boot to your windows 8.1 and to Legacy if you want to boot to your ubuntu).
I have done the same thing and my dualboot works like a charm with windows 8.1 and ubuntu 14.04
Link to download rufus for windows: http://rufus.akeo.ie/ (Get the latest version for best resaults)
I hope it works for you!