Apparently Nirsoft's ProduKey can do this:
produkey.exe /windir [Windows Directory]
Load product key information from
another operating system on the same
computer. The [Windows Directory]
specifies the base folder of Windows
installation, for example: c:\windows,
c:\winnt This feature is only
supported on Windows
2000/XP/2003/2008/Vista/7.
If that doesn't work, you'll have to dig it out of the registry file manually by searching
C:\WINDOWS\system32\config\software
for the registry key
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\DigitalProductId
using a hex editor I think.
Bytes 52-66 (0x34 - 0x42) of this key hold a 15 byte number.
In Hex, it's a 30 digit number:
A2 23 51 D0 2A 38 5D 22 C4 41 6B 87 43 C1 00
In Binary, a 120 digit number:
10100010 00100011 ... 11000001 00000000
Converted to base 24:
751AA001EHCCLAB3JH8KDIGAG
Mapped to Microsoft's custom base24 alphabet "BCDFGHJKMPQRTVWXY2346789" and a hyphen every 5 chars:
KHCQQ-BBCW2-TT7QR-F42M6-V3YQY
I did it this way about 4 years ago, so I'm really hoping ProduKey works.