Dear Linu Pathil,
There is no payroll with SAP-Payroll alone. That is a functional module in SAP named SAP-HR. That all all functionality of HR. It is actually a costly software.
I have posted lot of information to lot people in regards to SAP. Do check with the post. You cant learn SAP for free. It is like a ocean which can be covered only through proper channel...
Still have some specific doubts, post me a private message. Anyway All the Best!