my norton product key no longer works

Hello, I bought norton system works about 4 years ago to install on my home pc.  Since then, I have had a number of hardware related pc problems which has required my pc hard disk to be reformatted etc and has caused me to have to reinstall norton system works a number of times.  Now I have my latest rebuilt pc and I installed norton system works (actually just the anti-virus software) but I receive a message something like 'you have installed this product too many times and your product key no longer works'.

 

How can I overcome this problem so that I can properly register my norton product?

 

Thanks