The activation is tied to the computer's hardware fingerprint. If something big changes on the computer, the game thinks it's on a new computer and deactivates. (The memory is one of these things.) We are constantly trying to improve this to make it less annoying for people, but Culture Shock is still a little more finicky about activation than we would like.
You're not going to lose your ability to activate the game, don't worry.

Each game comes with 10 activations, and we're happy to increase this for people if needed. Our policy is that once someone buys a game from us, they can activate it as many times as they want.
The reason I asked about compatibility mode is because that error message doesn't normally show up in this situation. (Normally, the game would just revert to trial mode.) Click No on the error message, and then click Activation Support at the bottom of the screen and follow the instructions to get an activation key. I promise you nothing bad will happen.