If your 6-month-free-upgrade period hasn't expired yet, go here and enter the email adress you used to buy it in the first place.
If your 6-month-free-upgrade period has expired then you have to pay for Pro again. If you'd bought the CD version, poping the CD in the CD drive of your comp and installing it from there is doable, but you wouldn't be able to update your Pro version. You'd have to downgrade to Basic, and either update the Basics or buy Pro again.

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

"Raise your can of Beer on high
And seal your fate forever
Our best years have passed us by
The Golden Age Of Leather"
-Blue Öyster Cult-