The goal of Peak Fitting is available in Peak Analyzer only in OriginPro, not in Origin Standard. I presume you have Origin standard and not Pro?
The peak fitting ability used to be a separate module many versions ago, and it was added to OriginPro a while back.
You can upgrade your Origin to OriginPro, please ask your local Origin distributor for details. Also, in latest version 8.1 improvements have been made to Peak Analyzer including ability to perform batch peak analysis using a pre-saved peak analysis theme: http://originlab.com/index.aspx?s=8&lm=218&PID=1561#Batch_Peak_Analysis