If it is for Western European letters not used in English it will suffice to have the keyboard "United States-International" installed. You will find these letters by using AltGr or Ctrl+Alt in combination with letter keys. Thus the umlauts (AOU) are "hidden" under QPY.

If your needs go further than what is provided by the keyboard "United States-International" the program MSKLC (Microsoft Keyboard Layout Creator) will be helpful. MSKLC is free and can be downloaded from the Microsoft website. It allows you to define your own keyboard that can be used alongside with the standard keyboards provided by Windows. It even allows the use of dead keys and that really extends the possibilities of a single keyboard enormously.

MSKLC does not allow to alter the function of keys like Insert or CapsLock, so if you want to do that KeyTweak may be useful, as long as you bear in mind that every user of your computer will be stuck with the changes brought about with KeyTweak.