I use the US and United States-International (INTL) keyboard layouts, since I occasionally need to type characters not normally found in English. I rarely use the INTL layout intentionally. However, Windows is constantly switching to it. I am definitely not physically pressing the Win + Space key combination to switch layouts.
From what I can tell, this happens while I'm using the touchpad (Synaptics) on my laptop. I have problems with the Synaptics drivers interfering with games by briefly removing focus from them when I scroll, and occasionally sending a Ctrl keystroke, so it would not surprise me in the least if Synaptics is responsible. It never seems to happen while I'm typing, only between typing.
It's probably important to mention that I've been having this problem since the days of Windows XP Professional. I've never used anything besides x64 in the time since I started using multiple keyboard layouts. Most of my OSes come from TechNet or MSDN (retail, US), but I've also used betas, previews, and OEM releases. I tend to use the Professional/Ultimate versions. I've had the problem even after disabling the keyboard shortcut if I recall correctly, but it's been a while since I've done that; it's really not an option in Windows 8 anyway, since the taskbar isn't always visible.
I use Linux quite a bit. I want to say I've set the equivalent shortcut in GNOME to Alt + Shift and didn't have the same problem, but it's been a while. I still cringe whenever I hold Alt and Shift together, and I often repeat Alt + Shift if I perform an incomplete shortcut that uses those two keys out of habit. I'm pretty sure that keyboard shortcut isn't the issue.
