You must have a valid product key to install Windows. Your only alternative is to buy a brand new copy of Windows so it won't hurt to at least attempt to get an inexpensive replacement from Microsoft.
Determine if your copy of Windows is a retail copy or preinstalled copy:
Retail: Your copy of Windows is a retail copy if you or someone else purchased Windows as a standalone software package and then installed it on your computer. Your copy of Windows may also be a retail copy if it came on your new computer and your computer came from a small builder. Proceed to Step #3.
Preinstalled: Your copy of Windows is a preinstalled copy if it was already installed when you purchased your new computer. This is likely the case if you have a major brand PC and you've never installed a new copy of Windows yourself. See Step #2.
Other: If you purchased or were given a copy of Windows from your organization, business, or other group, see Step #2 but contact the issuing group instead.
Contact your original computer manufacturer directly to request a new product key if Windows was preinstalled on your PC.
If your computer manufacturer is not able to assist in issuing you a replacement product key for Windows, proceed to Step #3. Microsoft may still be able to help.
Call Microsoft at (800) 936-5700.
Note: This is Microsoft's Paid Support telephone number. Microsoft's site advises that support calls to this number will incur a $40 to $60USD charge. You will not be charged this amount for a call about a new product key.
Follow the auto-attendant prompts appropriately so you can speak to a customer service representative about your missing product key.
I was able to reach a human after only three questions - not too bad, in my opinion.
The Microsoft representative will take your contact information - your name, telephone number, and email address - and then ask for details about your problem.
Tell the representative that you have your original Windows installation CD/DVD but need a replacement product key.
Answer whatever questions that are asked of you. The Microsoft representative may ask for specific details about your Windows installation disc, like the numbers around the inner circle of the CD/DVD and details about what words or images that may or may not be on the disc.
Microsoft asks these questions to help verify that the installation disc you have is not pirated.
Microsoft will take your credit card information after verifying that your installation media is genuine.
This new Windows product key should cost you $10 USD, plus tax. A few readers have even emailed me saying that Microsoft didn't charge them anything at all so they're either changing their policy or some folks are just getting lucky!
The Microsoft representative will then read you your new product key and ask that you enter it into the activation window to be sure it creates a new installation code.
He or she will then transfer you to the telephone-based activation center to complete the Windows activation process.
Note: If for whatever reason you're not able to get a replacement product key from Microsoft or from your computer manufacturer, and your copy of Windows is not currently installed (excluding you from the product key finder method), then your final course of action is to purchase a new copy of Windows.
You can buy Windows 8 and Windows 7 directly from Microsoft or from popular online retailers like TigerDirect and Newegg. Older versions of Windows, like Windows Vista and Windows XP, are harder to find but there are still a few retailers that carry them.
Did you call Microsoft for a replacement Windows operating system product key and have a different experience? If so, let me know!
This article was based on several readers' experiences with the product key replacement process at Microsoft. However, if the process has changed, I'd like to know so I can keep this page updated.
Special thanks to Jay Kinnard for much of the information in this article.