DataFit

Origin can fit your function to your data

I recommend you MATLAB (as Adrian mentioned) or Maple.

More information on MATLAB:

http://www.mathworks.com

and Maple:

http://www.maplesoft.com

A useful thread on your problem can be found at Matlabcentral:

http://www.mathworks.com/matlabcentral/newsreader/view_thread/172926

Good luck!

This is also possible in R (http://www.r-project.org/)

You can also take a look at Scipy/Numpy (http://www.scipy.org/) for the Python language.

http://www.alglib.net/ is another option.

Maxima is an opensource alternative to Maple/Mathematica for symbolic math.

http://maxima.sourceforge.net/