That's right; position: absolute will be related to the top left corner of the screen normally.
However, if you put a position: absolute element inside a container that has position: relative, it will take its position from the top left corner of that container.
So if you make your wrapper position: relative and put the nav position: absolute inside it, the nav div will be fixed inside the wrapper, but if the wrapper moves (for instance centralising in different size windows), the nav will move with it but stay fixed relative to the wrapper.
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.