Had the same problem with DW8. I was in Texas and the disc was in Toronto
I tried "Control Panel>Add/Remove" click the program and click the change button then choose "Repair" from the Dialog box, did'nt work for me but worth a try. My solution was to uninstall, then download & install the "trial" from Macromedia's site. I guess the uninstall leaves the serial info in the "Registry" or something, not sure but everything is working just fine. Hope this helps and good luck.
it's so simple
Go to : C:\Users\[Current User]\AppData\Roaming
Or Windows start menu > Run > type: %appdata% and hit enter.
From there, navigate to Adobe\Dreamweaver CS(Version Number)\(Language)\Configuration
In my computer it’s Adobe\Dreamweaver CS5.5\en_US\Configuration
Find theWinFileCache*.dat file(s) and delete them.. If you have more than one Configuration folder (I have "Configuration" and "Configuration-1") go into each one of those and delete WinFileCache*.dats.