You can look up the version of any Windows installation in the registry.
Therefore I would mount the SOFTWARE Hive of both installations from within a working Windows installation. It is located at %SystemRoot%\System32\Config\Software (%SystemRoot% being the respective Windows main folder) as explained in this msdn article.
To mount it open regedit from your working Windows installation, hightlich HKEY_LOCAL_MACHINE and chose File --> "Load Hive..." from the main menu. Browse to the Config folder, and choose the Software file (you have to have all files set to be visible in the explorer settings for this to work). After selecting the Software file, give the new subhive a meaningful name so you can distinguish it later. It will be created as subkey of HKEY_LOCAL_MACHINE under the name you chose. You can browse it like any other key. Go to \Microsoft\Windows NT\CurrentVersion under your new subkey. There you can find all information about the Windows Version you need ( Name, SP, Build, etc).
After you are finished unload the hive again via the main menu.