Problem entering serial number

Hi!
I bought version 6.20 some time ago and would like to upgrade to v7 but the serial number I am entering isn't working, how do I find my correct serial number if I can't find the CD?
Many Thanks!
I bought version 6.20 some time ago and would like to upgrade to v7 but the serial number I am entering isn't working, how do I find my correct serial number if I can't find the CD?
Many Thanks!