Installed new windows but lost the windows key - Laptop

Hi,
My brother has sony viao laptop,
i had live usb windows, he accidentally installed it, he dont know about computers much, he only use facebook.

Problem is that we dont have the key and neither he saved the key anywhere for the previous windows installation.
The previous windows is also inside the laptop.. with

Code:

windows.old

folder name.
They annoying thing is he did that installation again and again thinking it might fix problem, i dont know what was he thinking.

So there are three directories.
windows.old
windows.old.000
windows.old.001

so there are 3 old installations in that laptop..

The genuine window is inside the windows.old.

How to get the key from windows.old folder??

or is there any other way. i dont want to buy another key or want to use crack. Please help me get the old windows 7 key back..

ok. i got the key from windows.old directory using this software ProduKey - Recover lost product key (CD-Key) of Windows/MS-Office/SQL Server.

Now how to roll back to the old windows??