[prewett]

If I recall correctly, Win32 let you stuff a 32-bit value into a window. If you wanted to write an object-oriented application, you'd stuff your this-pointer in that 32-bit value. Since every little thing (button, label, checkbox) is a window, you'd have a lot of that. Visual Studio might be old enough that they wrote the pieces that are still 32-bit in straight-up Win32, or maybe MFC. If so, I'd bet no one wants to touch it to port it over to modernity...

[mizaru]

You're probably referring to SetWindowLong. A 64-bit compatible version (SetWindowLongPtr) of that function is available since Windows 2000.

I'm pretty sure there's a guide on how to properly port applications to 64-bit on MSDN somewhere.