What could be causing Vim to switch keymaps randomly?

I've noticed a problem with Vim, where the keyboard mapping would unexpectantly change (to French I think, but I'm not sure). For example, the character 'É' appears when the key '?' is pres开发者_StackOverflowsed.

My keyboard is set to English, and I don't have any other languages on my computer.

Restarting Vim fixes this problem temporarily, but the problem reappears after a while.

What could be causing this, and what can I do to fix it?

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I found this blog post on the same problem. Apparently left-alt + shift will do it. Removing that from the Windows "Advanced Key Settings" dialog and/or removing all unwanted keymaps should fix the problem.

I am not on a Windows system at the moment so cannot verify this.

Update

I have tried it on a Windows system and can verify that this is the problem - alt+shift defaults to cycling through the all the keyboard configurations.

It can be changed from (deep breath...)

Control Panel -> Regional & Language Options -> Languages Tab -> Details button -> Key Settings -> Switch between input languages

Unselect both tickboxes on the final dialog.

I find it astonishing that anyone at Microsoft thought it would be a good idea to have a simple key combination that silently changes the keyboard mapping and other language settings, and only for the current program. How often has anyone wanted to do that?