Another late reply but the above (apparently now "below" answer is still the best anwser according to Google search results (It should be awarded "best answer"). I should add that it depends on the mouse you're using. I'm using a Logitech M100 and it doesn't exhibit this quirk, but if I attach one of my other gaming mouse/mice to my computer (A Zowie S2 and a Microsoft Intellimouse Pro) it has this problem. It's easy enough to solve, you just have to figure out which keyboard HID device to disable, since apparently there can be multiple "HID Keyboard Device" entries in the device manager!

I just registered here today, I'm not sure if it's encouraged to answer very old threads in StackOverflow, so apologies in advance!