Did you buy the physical disk? It will be included with the packaging.
If you registered the software with Adobe, then you can log in to adobe.com and find it.
Can I use the same number to install Dreamweaver on my second machine at my holiday home?
Yes, you are entitled to a maximum of 2 activations of the software on one license key.
first of all you could search in your WIN Registry. More Adobe help here:
Or you have a look here: "How Do I Find My Dreamweaver CS4 Serial Number?" ( If you've lost your serial number, there may be a way to find it as long as Dreamweaver CS4 is still installed. You might be able to find your Dreamweaver CS4 serial number if you've uninstalled the program but only if the serial number information was left in the Windows Registry.) This program wil help you, here the link to it: