Ruby Programming/Standard Library/Win32API
The Win32API is basically an easy way to designate the parameters and return value of a function in windows (for example, one contained in user32.dll), and call it.
Note that Win32API is deprecated in Ruby 1.9.2, but there will probably always be a Win32API gem around, or an ffi wrapper to it.
Note also that you may as well just use ffi, which has many windows examples available. Another option would be to use DL.