How to re install Photoshop CS6 after Windows update trashed my system

I have Photoshop installed on a Windows Vista PC. A recent Windows update left the machine un-bootable. PC World had a look at it and advised that Windows needs to installed from scratch an that all applications would need to be reinstalled. My concern is that I don't have the original Media or serial number. Photoshop was originally installed by the company who sold a complete package including a Canon large format printer.  I think it was originally CS3 but may have been upgraded to CS6. This is my brothers PC and I have been given the task of recovering it for him.  My brother isn't very technically minded (he got the creative genes) and can't remember if he was using CS3 or CS6.

I have removed the hard disk and put it in a Windows 7 system and I can access the drive and I can see there is a Program Files\Adobe\Adobe Photoshop CS3 folder.  However there is also an Adobe CS6 setup folder on the disk.

So my questions are:

1. How can I be certain what version of Photoshop was installed and running?

2. Can I download the Photoshop CS3/CS6 install files?

3. Can I retrieve the Photoshop serial number from the hard disk?