hi
i was in exact same position, trying to get davinci to work after buying credits ages ago and now having this issue. anyway, with a quick "flash.ocx vista" google search i found its not isolated to just this client program. seems to be an issue with different versions of flash relating to the browsers or something. regardless, i downloaded and installed flash from this address:
http://www.softwarepatch.com/internet/flash.html it seemed to fix the problem, must have the .ocx file. though flash then stopped working in firefox and i had to reinstall it yet again through the browser. so its not really a permanent solution.