How can I determine what version of Windows in the windows.old folder?

In C:\windows.old\system32, open the properties for any system file, like 'ntoskrnl.exe'. On the Details tab, you see the Product Version:

• 10.0 = Windows 10
• 6.3 = Windows 8.1
• 6.2 = Windows 8
• 6.1 = Windows 7
• 6.0 = Windows Vista
• 5.2 = Windows Server 2003
• 5.1 = Windows XP
• 5.0 = Windows 2000
• 4.0 = Windows NT 4

It gets a bit trickier if you need to know whether it was a home,pro, enterprise or server version.

Open the "\Windows.old\system32\license.rtf" file in a text editor and look at the 2nd title row, it will tell you the exact Windows edition, as was initially installed. I verified this method on Windows Vista Business, Win7 Ultimate and Win8.1 Pro.