Which operating system ???
If you are trying to recover a key of an already installed program on your computer there are freeware keyfinders on the internet
http://download.cnet.com/%7Bvalue=http://www.download.com/magical-jelly-bean-keyfinder/3000-18487_4-10079600.html The Magical Jelly Bean Keyfinder is a freeware utility that retrieves your Product Key (cd key) used to install windows from your registry.
It has the options to copy the key to clipboard, save it to a text file, or print it for safekeeping.
Serial keys Location
Lift up your computer and look for a tag.
The tag will probably be located on the side of the computer, or on the bottom of the unit for a laptop.
The service tag is generally a large black and white tag and it will say "Service Tag Number."
Do not confuse this tag with the Windows tag, which may be located in the same area on the computer.
or
Restore My Product Key
Product key glitches can occur as a result of operating system file changes and random errors, leaving you with constant re-registration reminders even where you have already entered a valid key for a particular product.
The quickest fix is to restore your computer using the System Restore feature, which will take your computer's settings back in time to a point before the product key problem first occurred.

Open "Start," click "All Programs," open "Accessories," and click the "System Tools" folder.
Click on "System Restore" and select the option to "Restore my computer."
Click on a restore point from before the product key error first appeared.
Press "Next" to begin the automated restore.

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Get a New Product Key
Having the product key of a software application is essential when installing and activating the software on your computer.
Product keys may also be called CD keys, serial numbers or activation codes.
If, for some reason, you lose the product key, or it stops functioning, you must get a new product key from the creator of the software.
Most software companies work with their customers in resolving product-key issues.
For example, Microsoft advises its customers to contact Microsoft Support Services in the event that a product key becomes lost or damaged.

Go to the website for the software company.
Look for a heading such as "Contact Us," "Customer Service" or "Support."
Get the contact information for the company, such as an email address or telephone number. Some companies require that you contact them through a web-based form on the website.
Contact the company by phone or email explaining that you have lost your product key or that it is no longer working.
Include as much information as you can to help them verify that you are an authentic customer. Include copies of receipts you received (email or physical copies).
If you have original packaging from the product, include any relevant bar-code or version numbers.
Refer to the name and email address that you used to register the product.