If, for some reason, you have trouble installing Windows 8.1 Update using your existing Product ID key, you can try using one of the keys listed in the following Microsoft TechNet Library article: Appendix A: KMS Client Setup Keys.
There are, however, two important limitations you should be aware of when using one of the PID keys from the above article:
- First and foremost, the KMS Client Setup Keys provided by Microsoft in the above article only allow you to install the operating system. Once the operating system is installed, the computer will boot to the Desktop, but Windows will not be activated/unable to activate using Microsoft-provided PID key. You will need to enter your own legally-licensed PID key to replace the one used from the above article. Basically, the keys provided in the article are for getting you through the install process should your existing PID key not validate correctly at install time.
- The Microsoft-provided KMS keys only work with business versions of Windows operating systems, such as Windows Vista Business, Windows 7 Professional, Windows 8 Professional and their various Enterprise SKUs. If you have a home edition of a Windows version, like Windows Vista Home, Windows 7 Home Premium, Windows 8, or one of their various Ultimate SKUs, the PID keys won't work.
Hopefully, you won't need to use one of the keys from the above article to get through the install process for Windows 8.1 Update, but if your Windows 8 Professional PID key doesn't work, this should allow you to get up and running to the point where you can get the key correctly entered, or at least contact Microsoft support for help activating your copy of Windows.