It’s easy with virt-cat:
$ virt-cat YourWinGuest /WINDOWS/system32/prodspec.ini ; ;Note to user: DO NOT ALTER OR DELETE THIS FILE. ; [SMS Inventory Identification] Version=1.0 [Product Specification] Product=Windows Server 2003, Enterprise Edition Version=5.0 Localization=English ServicePackNumber=0 BitVersion=40 [Version] DriverVer=10/01/2002,5.2.3790.0
There’s not much documentation about the “prodspec” file, except that we know it gets written after an install and after a service pack update.
From what I can tell, BitVersion seems to be the level of encryption enabled (ie. 40 bit for export or 128 bit for US copies).
No news on whether this is a 32 or 64 bit version of Windows itself — the above is from a 32 bit version.
Richard WM Jones 
Sadly, not documented and not present on other versions of Windows (I am using 64bit Win2k8) 🙂
Ick … well we can also grab the Windows Registry and get it from there, also using virt-cat …
https://rwmj.wordpress.com/2009/06/08/explore-the-windows-registry-with-libguestfs/
Note references in the above to “guestfish … download” can now be replaced by a simpler invocation of virt-cat.