What is the difference between a Windows key and a CAL?

jamesbcox1980 used Ask the Experts™
We have a windows small business network.  The computers came with Vista, but someone installed XP on them and attached them to CALs on our SBS server.  We purchased 15 CALs, but I can't find any invoices of someone purchasing any Win XP licenses.  Does a CAL mean that you're licensed, or do you still need a license key and you're just allowed to access the server?
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You need both. The CALs are Client Access Licenses, separate from the licenses and keys for XP or any other desktop OS.
A Client Access License ("CAL") is a software license distributed by software companies such as Microsoft to allow clients to connect to its server software and use the software's services,A Client Access License (CAL) is a kind of software license that legally permits client computers to connect to Microsoft server software. They usually come in the form of a Certificate of Authenticity (CoA) and a license key, which is sometimes attached to the certificate itself.
If you've lost your product key, it's located in the registry but is encrypted and not readable, making finding it difficult. The key was not encrypted in previous versions of Windows like Windows 98.
Follow the steps below to locate your Windows XP product key code.

1.Manually locating the Windows XP product key from the registry is nearly impossible due to the fact that it's encrypted.

Note: The manual techniques used to locate the product key for operating systems like Windows 95 and Windows 98 will not work in Windows XP. Those manual procedures will only locate the product ID number, not the actual product key used for installation. Lucky for us, several free programs exist to help find product keys.

2.Choose a free product key finder program that supports Windows XP.

Note: Any product key finder that locates Windows XP product keys will locate the product keys for both Windows XP Professional and Windows XP Home editions.

3.Download and run the key finder program. Follow any instructions provided by the software.

4.The numbers and letters displayed by the program represent the Windows XP product key. The product key should be formatted like xxxxx-xxxxx-xxxxx-xxxxx-xxxxx - five sets of five letters and numbers.

5.Write this key code down exactly as the program displays it to you for use when reinstalling Windows XP.

Note: If even one character is written down incorrectly, the installation of Windows XP that you attempt with this product key will fail. Be sure to transcribe the key exactly.

A windows key is the Operating System license for the machine that it is loaded on. A CAL al(Client Access License) allows named users / devices  to connect to a server...

A Windows Server CAL is required when a user or device accesses or uses the Windows Server server software. However, if access is through the Internet and is anonymous (for example, when browsing a public Web site), a Windows Server CAL is not required. Also, if that user is an external user, another option is to acquire an External Connector license.

(If the PCs came with OEM licenses, downgradeable to XP, is it possible you may not have a separate invoice...)
Windows Key - This key needs to install Windows XP / VIsta/ Server 2003 etc.. to your computer/server. Usually this key stick on outer cover of your computer. Or you can find it in installation disk pack.

Client Access Licenses (CALs) - required to access server services. (Required if you have a domain environment) Usually SBS server comes with 5 CALs.

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