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.

Best would be a solution that I could implement in Excel, but anything else is fine too (preferably one that could be rutinely imported into Excel).