From Ross-Tech Wiki
5.2 What's the De-Activate button for? Why would I want to do that?
To move your activated copy of VAG-COM permanently from one PC to another. See the previous question. *
Some things you might want to do to your PC will kill the Activation. If you're going to re-install Windows or update to different version of Windows, re-format or re-partition, or replace your hard drive, then you're going to loose your Activation anyway, and you'll need to ask us for a new activation code when you're done. Sometimes we can tell from the Serial Number that you're requesting a new activation for the same PC. Sometimes we can't. It depends on exactly what you've changed. Now you can always get a new Activation with no questions asked IF you are a registered Ross-Tech customer and you supply proof of de-activation from a previous installation. So if you're about to undertake surgery on your PC, please use the De-Activate function first.
To De-Activate, simply go to the About Screen and press the Register/Activate/Transfer button. Next you will see a button labeled as "De-Activate". Press the De-Activate Button. Doing so will generate a "Proof of De-Activation" code which you can copy and paste into the appropriate Registration Form using the serial number from the About screen. The "Proof of De-Activation" code is also written to a file called DE-ACT.TXT which you can easily put on a floppy disk for safe keeping.
Note: A De-Activation code cannot be used to activate a new installation of VAG-COM! You must fill out the appropriate Registration Form to receive a new Activation Code.