I'm using Windows 8.1 and the easiest way of dealing with this is by using a little VM with windows 2000/XP. In my case I use this for EMS carts as well as for by Xchanger 2 (via a USB parallel port). You may choose to use windows 2000 as there are compatible drivers and the install is a bit smaller but I choose windows XP. Using the free Virtualbox software I created a VM, installed the drivers and utilities and then create a memory image so that there was no need to wait for the VM to boot every time I wanted to use it.
Otherwise you can enable the test signing but if this is the only unsigned driver that you're planning on using then that probably not the easiest or best way of doing things (you'll need to include and sign a .cat for the .inf and the run the bcdedit -set TESTSIGNING ON to load the drivers). If you are using lots of older or unsigned drivers then you might want to set up your computer to always allow test signing. DSEO won't allow you to go into test mode but it's signing functions seem to work fine with Windows 8.1.
EDIT: There are also experimental linux drivers available. Last I saw they were incomplete but that's an avenue worth looking into. A linux VM could be exceptionally small (Damn Small Linux is about 50MB) and you could further strip any software you don't plan on using, turning it into an environment designed purely for interacting with EMS carts.
DOUBLE EDIT: Looking at the EMS Flasher for Mac / Linux, it should be possible to port over to a Windows machine given an install of Cygwin, as it should run on any system with libusb.