Taken from the Linus Tech Tips site:
http://linustechtips.com/main/topic/172028-how-to-fix-steelseries-firmware/
1. Remove all steelseries software from the computer (Engine, firmware updater, ect.) Note: if you have both steelseries engine 3 and steelseries engine 2 (just called steelseries engine) just remove the one that pertains to what you are fixing
2. Make sure there is no steelseries software running (task manager > processes) if there are any, hit end process
3. Plug in your extra mouse/keyboard, then remove all outdated drivers from old products (this is a good thing to do anyway) (device manager > view > show hidden devices, then go down to mouse/keyboard and right click > uninstall do this to all the old drivers), also uninstall your steelseries mouse/keyboard (I was able to tell my two mice apart because my spare had power saving settings, but the steelseries did not (as it was not fully functional), also, old mice will be grayed out, connected ones will not be)
4. Turn the computer off, then plug in the mouse/keyboard
5. Turn the computer on
6. Let windows do its thing for a little while with the drivers (it will probably fail, mine did)
7. You should now have at least some functionality with the mouse (can move it and click, but maybe not use scroll wheel/click and drag), if you have full functionality that is fine
8. Download the steelseries engine
9. Go through the installer, it will ask you to update firmware, say yes
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I'll keep poking around..