I don't know if it will work out, but if you own CS2 then you qualify to get the special version that is available now. Maybe you can install that and then CS3 will recognize it is installed.
As is, if you are trying to install your own version of CS2 you would not be able to anyways. There is no activation server available. You can read about it and how to get the special version thru the following link:
Error: Activation Server Unavailable | CS2, Acrobat 7, Audition 3 -
Thanks for the (extremely prompt!) answer - I've marked as "correct" as I reckon it would work (the installation dialogue box only required a valid CS2 serial and the one on the download link - 1130-0412-8377-1896-9751-5759 - should work, according to what the page states).
Fortunately I managed to find (an old sketchbook that I was using at the time that had a note of both my CS2 and CS3 serials so managed to finish my install from that... one advantage of hoarding old notebooks !
Thanks again, and much kudos to you !
NB. If anyone else has the same issue (re-installing the upgrade version of CS3) then there is no need to install CS2 first before installing CS3 - the system should know you had a valid CS2 and only requires the valid serial.