HKEY_LOCAL_MACHINE\SOFTWARE\City Interactive\Sniper2

The original Sniper Ghost Warrior has been re-released on (Good Old Games) in a DRM-free version. GOG removes all activation requirements. For $5-10, you can own a version that never asks for a serial key. This is the recommended solution if your original key is from a defunct bundle site.

it is usually due to missing or corrupt Windows Registry entries, or a mismatch in how the game recognizes its own installation 1. Create a Registry Key (Most Common Fix)

"If the key fails, the ghost isn't ready. Look behind the physical."