SetDirectory["C:\\Documents and Settings\\Chemistry\\Desktop\\DLSstudents"]

dataout=Import["d1out.txt","Table"];

gdataout=ListPlot[dataout,PlotJoined->True]

Find 
k, T , viscosity, and n refraction index (see textbook)

Calculate 
q (textbook) for 90 dgrees and the wavelength used   (say 633nm HeNe)

Calculate 
D for 100nm Radius and put a factor f (or 1/f)  as a fitting parameter

Make sure 
the time units 20 microseconds are included

Do the fit  (similar to the textbook)

fitdataout=NonlinearModelFit[dataout,aa+1+BB*Exp[-2*dqq100t*ff*t],{aa,BB,ff},t]
Extract results and errors

fitdataout["BestFitParameters"]

fitdataout["ParameterErrors"]

the ff value is th efitting parameter that can give you the required R value

Plot experimental  data and modelfunction

Present R with error (propagation of error)