Dear Linu Pathil,
There is no payroll with SAP-Payroll alone. That is a functional module in SAP named SAP-HR, which encompasses all the functionality of HR. It is actually a costly software.
I have posted a lot of information to many people regarding SAP. Please check the posts. You can't learn SAP for free. It is like an ocean that can only be navigated through the proper channels.
If you still have specific doubts, please send me a private message. Anyway, All the Best!