If you already purchased a Registry Booster copy, but the program is still asking you to purchase, this is because you still have to activate your copy.
Activation is simple:
1) Open Registry Booster
2) Click on the orange "Register Now" button in the main program window
3) Enter your serial number (a long number found in your Order confirmation email and starts with the letters: "RB-")
The above procedure will unlock the full version of the program.