What operating system are you using?
Where did you buy Lightroom?
Have you ever had Lightroom installed and running?
What is the exact message that you receive?
I don't believe it's possible for the installer program to know that you have installed Lightroom "too many times". Your serial number should have six groups of four numbers, and the first four should be 1160. Please do not post your serial number on this website. But if you can provide some more details someone might be able to help you get things going.