Recovering a Windows XP License key

If the old machine had a sticker, it was most likely an OEM license.

OEM licenses cannot legally be moved from one computer to another one. They die when the computer dies.

But, in answer to your question...

Load the old registry hive on your new computer and then find the XP key. -- EDIT make sure that you open the SOFTWARE hive instead of SYSTEM mentioned in the first guide, then open this just-opened-offline hive in the second guide.

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

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.

WinKeyFinder - · View Windows XP CD Key · Change Windows XP CD Key · View Windows Vista 32bit Edition CD Key · View MS Office 2007 CD Key · View/Change XP Key · View/Change MS office Key · View Win98 Key · view Win2000 Key · View .Net Key · View Windows2003 Key

Magic Jelly Bean Keyfinder works on Windows 95, 98, ME, 2000, XP, Vista, Server 2003, Server 2008, Office XP, Office 2003, and Office 2007 family of products.

and while you're at it, backup your XP activation files, AFAIK the latest WinKeyFinder features this option, if not, here'a a tutorial.

This article might be useful: Find the Windows XP Product Key Code.
There is also this another link on a ViewKeyXP tool: Retrieve your XP Product Key.
(that second link has some other tool references too).