There are many ways to enter these characters, choose the one that works best for you.
Method 1:
In many Microsoft Office applications you can use the following:
Press three keys (Ctrl-Shift-colon), release then type the letter you want with an umlaut ('a' for example)
New! Microsoft Office 2003 International Character Toolbar.
This add-in enables you to insert international characters from 26 languages into Microsoft Office 2003 programs.
Method 2:
In virtually all other places in Windows, you can use the 'native' method of entering the characters. Hold down the (Alt) key and enter the following number using the numeric keypad on the right of the keyboard (you must include the zero).
character keystroke äAlt-0228 ÄAlt-0196 öAlt-0246 ÖAlt-0214 üAlt-0252 ÜAlt-0220 ßAlt-0223 €Alt-0128
Method 3:
A third method that works in some other applications (including Dreamweaver) is as follows:
character keystroke äAlt-132 ÄAlt-142 öAlt-148 ÖAlt-153 üAlt-129 ÜAlt-154 ßAlt-225