I'm getting out of my depth here, but I'll try to answer the parts that I understand..
1. Should I use the spin button from Form Control or Control toolbox?
I have no idea. Perhaps they are in fact the same control. I haven't done anything with userforms in Excel, so don't have experience with the controls. If they have similar functionality, it may not matter which you use.
2. I am talking on the basis of the Finnacial model which contains about 20/25 sheets of data and at at the end only we can make graphs of certain variables like change in IRR, NPV, Net Profit. If I change one variable the result will be for the new one. I want to show the new one as well as the old one in the graph to make comparison. pl help me on this
I haven't used VB to drive much in Excel, but certainly a chart can include more than one data series. That's probably what you want. My guess is if you can figure out how to add or modify one series from VBA, it shouldn't be too difficult to add another.
3. Then I want to take this graph sheet as a picture into the PowerPoint presentation without data sheets but with the spin button. So when during the presentation, if I change the values through the Spin Button in PowerPoint, it should change the graph in the PowerPoint itself without going back to the Excel work sheet. How can be this done.
To be honest, I'll be a little bit surprised if this is possible, But don't let me put you off, I have no experience with PowerPoint.
It sounds to me as though you should try searching the web for tutorials on this stuff. If you were trying to do it and getting errors in your code, TheScripts would certainly be the place to find answers. But to find out how to do this stuff from scratch, I don't know whether we have anyone who can help out, or not.