When you say it refuses, what does it do? Any messages - exact wording please?
Did you download the latest LR3 version from Adobe or do you install from your original media?
Please also note that if your LR3 serial is an upgrade serial you will also need the serial from LR1 or LR2.
What is your OS?
Old computer = Dell Precision 6600
New computer = Del Precision 6800
As close to the same thing as possible
OS on both = Windows 7 pro
Loaded LR3 on old computer from original CD.
Entered serial number from LR2 and LR ran.
Old computer died.
Loaded LR3 on new computer
Entered serial number from LR2 and got a big red X
Tried 2 more times - same big red X
Double check that the serial number was correct.
I figured out the problem and solved. I knew I had to enter the serial number on LR2 as this was a update. I just entered the serial number in the wrong place.