See the question and my original answer on StackOverflow
  • The main exe "Start Menu Calculator.exe" installs a windows hook (using SetWindowsHookEx) into explorer.exe. The hook is implemented as usual in SBLib.dll which is then injected into Windows Explorer's memory space.

  • This hook searches for window handles belonging to the search box. See a discussion around this here: How do I get a "handle" on the Windows Search textbox? and probably sub classes the search box windows (if you kill the "Start Menu Calculator.exe" process abruptly, it crashes Windows Explorer too... which kinda confirms this)

  • It then reacts to key presses, and I suppose it butchers up the result window. In the hierarchies of Windows, I think it's a Window named "Desktop Search Open View", you can get to it with SPY++ under "Start Menu", aside the windows mentioned in the msdn forum above.

So, no nice API behind this nice application. Massive hacks instead :-)

I think however, some level of integration is possible, using documented behavior, with the search box. I have not dug further, but there is the notion of federated search in Windows (Windows 7 Federated Search). I don't see if this would be capable of reacting instantaneously to what the user types in though...

As a side note, if you're also looking for a way to run javascript code from C#, there is a question here on SO that says it all: parse and execute JS by C#