Reduced Functionality mode After the grace period, if you have not entered a valid Product Key, the software goes into Reduced Functionality mode. In Reduced Functionality mode, your software behaves similarly to a viewer. You cannot save modifications to documents or create new documents, and functionality might be reduced. No existing files or documents are harmed in Reduced Functionality mode. After you enter your Product Key and activate your software, you will have full functionality for the programs and features that you purchased.

As you probably know (because you've found yourself here), you must have a valid product key to reinstall Microsoft Office 2010 or Office 2007.
If you haven't already looked, you can avoid the process below by checking for the product key on the disc sleeve, manual, or email receipt that came with your purchase of Office 2010 or 2007.
Beyond that, assuming Microsoft Office is still or was recently installed, the valid product key you need to reinstall Office is stored in the Windows Registry. Unfortunately, digging it up from there won't be much help because it's encrypted.

Ms Office 2007 Pro Serial Key

Fortunately, several free software programs called key finder tools are more than capable of finding, and decrypting, that super important Office 2007 or 2010 product key.
Product
Follow the steps below to use the free LicenseCrawler program to find and then show you your valid Microsoft Office 2007 or Office 2010 product key:

How to Find Your Microsoft Office 2010 or 2007 Key Code

The following procedure works equally well to find the product key for any Microsoft Office 2010 or 2007 suite, like Office Professional 2010, Office Professional Plus 2010, Office Ultimate 2007, etc. These steps will also work even if you just have one member of the suite installed. For example, 2010 or 2007 versions of Word, Excel, Outlook, etc.
  1. Download LicenseCrawler. This is a free, and portable (no installation required) program, as well as one that I've tested for valid product key extraction for both Office 2010 and Office 2007.
    You're welcome to try a different free key finder program but we like LicenseCrawler for Office 2010/2007 product keys the best, plus we love that it's portable and leaves nothing behind on your computer. It's not like you're going to use this program twice.. hopefully not, anyway.
  2. After downloading, extract to a folder the contents of the ZIP file, and run LicenseCrawler.exe.
  3. Read through the screen prompt you see now, and press the button that it tells you to press.
    There might also be an advertisement or some other initial setup screen (like a language selection or license agreement) that you have to wait for, close, or click through to continue. Just follow any on-screen instructions to open LicenseCrawler.
  4. You might see another window open that's unrelated to LicenseCrawler, but it should close down on its own and not interfere with the product key scanning procedure.
  5. Wait for LicenseCrawler to scan your entire registry, looking for registry keys that contain product key information. Since you probably have many more programs than Microsoft Office 2010 or 2007 installed, you'll probably see lots of entries.
  6. Once LicenseCrawler is done scanning the registry, scroll down through the list and look for the entry that starts like one of these:
    Here's what you need to know about these entries:
    • 14.0 refers to Office 2010
    • 12.0 refers to Office 2007
    You'll only see one unless you happen to have both versions of Microsoft Office installed, but that's not common.
  7. Under that entry, note the two rows, one labeled Product ID, another labeled Serial Number.
    The Office 2010 or 2007 product key is the alphanumeric series listed after Serial Number. The Office product key will be formatted like xxxxx-xxxxx-xxxxx-xxxxx-xxxxx. It'll be 25 characters long — five sets of five letters and numbers.
    The term serial number probably isn't the best way to describe what this number is, but you'll often see the terms serial number and product key used interchangeably.
  8. Write this product key code down exactly as LicenseCrawler shows it — you can either do this manually or copy it right out of the program. If you're off by even one character, it won't work.
    Another way to save the product key is to go to File > Save to make a text file that contains all of the results. You'll have to sift through them again to find the MS Office product key, but at least you'll know it's forever saved.
  9. You can now reinstall Microsoft Office 2010 or 2007, using the product key that LicenseCrawler showed you.
Unless your edition of Microsoft Office allows for simultaneous installations on more than one computer, please know that most of the time this is not allowed. Just one computer at a time.

Tips & More Information

If the above 'trick' didn't work, and you're sure you don't have your email receipt or other documentation available from when you bought Office 2007 or 2010, you're left with having to purchase a new copy of Microsoft Office.
While you may have come across various free Office product key lists or may have seen suggestions to use keygen programs to create a product key that will work, neither option is legal.

What About Office 2019, 2016, or 2013?

Unfortunately, the above process does not work with Microsoft Office 2019 down through 2013. Microsoft made changes to the product key process beginning in version 2013 that made it possible to restrict the storing of the key on the local computer to nothing but the last five characters, making product key finder programs unhelpful.