One cool thing about Windows is that although the path contains a space, it still runs the application fine, even if you don't place quotes around the entire command and even if you don't use the executable extension for iexplore.exe.

But how does Windows know where the program path ends and the program's command line parameters begin? How does it know that the user isn't trying to run a program named "C:\Program.exe" with the parameter "Files\Internet Explorer\iexplore?"

The problem is that it doesn't know. It just starts at the beginning and tries finding an executable until it finds a match. So in this case, it will try these files every time you run the command:

C:\Program.exe
C:\Program Files\Internet.exe
C:\Program Files\Internet Explorer\iexplore.exe

You might see where I'm going with this: if you place an executable named program.exe in the root directory, it will probably end up running quite a bit. In fact, it will run anytime Windows launches a Program Files executable that does not have quotes around the path.