Detect texbox focus for virtual keyboard

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C++

C

C#

Windows

I'm developing a generic on-screen keyboard in Windows (XP, Vista, 7). I need to detect when the user has focused on a textbox to automatically show the keyboard, and hide it when textbox loses focus.

Is there anyway I can achieve this? (The textbox is from other applications that I do not have codes for. I'm looking for a solution that can work for most generic applications).

Thanks for any help or pointers.