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 1-100.

D should be in the range of 100-500.

E is a constant.

Best would be a solution that I could implement in Excel, but anything else is fine too.