For anyone who wants to ditch Winblows and boot into GNU/Linux from a USB connection but their PC does not support USB booting, I recommend Plop boot manager
What this application does is when you start your computer, before you see the Windows boot logo, you will be given an option to boot into Windows, or to boot into PLOP manager. After you boot into PLOP manager, then you will be given the option to boot via USB.
To install this, first download the file (as shown in the link), to install you would want to go to plpbt-5.0.15\plpbt-5.0.15\Windows and click on InstallToBootMenu.bat. Follow the prompts, install it and enjoy PLOP
I have tested this on Windows XP and Windows 7 and it works fine on both Operating Systems.