What methods are there to fit a model of the form $y=A\mathrm e^{Bx}+C\mathrm e^{Dx}+E$?

Here is the actual scientific data to be fitted: http://dl.dropbox.com/u/39499990/Ben%2C%20real%20data.xlsx

B should be in the range of -1 to -100.

D should be in the range of -100 to -500.

E is a constant.

This specific model is of interest as it is the acceptable one in the scientific community for describing the biological proccess in hand- Inactivation of a voltage dependent calcium channel. (for reference see for example: A novel molecular inactivation determinant of voltage-gated CaV1.2 L-type Ca2+ channel. A Livneh, R Cohen, and D Atlas; Neuroscience, Jan 2006; 139(4): 1275-87. " The rate of inactivation was analyzed by a biexponential decay -A1exp(-t/Tao1)-A2exp(-t/Tao2)+C " )

Best would be a solution that I could implement in Excel, by the use of build-in functions or VBA code.