Can't re-install DMF5

[IanT]

Originally purchased version 3 in the box, have upgraded to 4 then 5. I have had to uninstall the program as it stopped working. But I cannot re-register or activate the product. Have used my original serial number to no avail, and tried to re-activate the TBYB version with activation code that was sent to me. Nothing works. Please can someone help.