Real Text Size Calculation?

I've got a small problem here. I'm trying to write some code that would
generate a drop-down menue for me, where I'd just need to enter the
menu- and submenu items into an array. The items are to be displayed as
text. Now, I want to have the layers with the submenues to appear more
or less under the appropriate main menu items. For this I need to be
able to calculate how many pixels a given text uses up on the screen of
the user. That's all fine with IE and an unproportional font like
courier. The problem is, there is this nice setting that allows you to
change the diplay size of the text - which is pretty much disfunctional
in IE, but in mozilla, it works. Which means that I can't just say that
one letter in courier, size 2, is 4 pixels wide and be happy with my
calculation, because then somebody can just pop up with another text
diplay size and *boom* - my calculation is wrong again.

Now, is there a realistic way to find out how many pixels _exactly_ a
piece of text takes up on the user's screen? Or can I make the given
piece of text a DIV and then have some kind of document.divname.left
that gives me the actual location of this text?