In general, the issue with the older CS apps is in reinstalling said software as it needs to go out to the server to validate the activation key. Said servers do not exist anymore. Upgrading an OS in place doesn't erase the existing licensing unless you do an erase and install. Whether the OS will actually run the software depends on the OS.
That being said, you don't have the activation keys since you purchased a computer with said software preinstalled. You would need the original activation keys even if you had the original installation software. So, in a nutshell, this setup of yours only works because you haven't needed to reinstall the software.
From a legal standpoint, you can't resell Adobe products. So, you don't actually own a license to use the software, you are using the license granted to someone else. Which is why you are stuck in the situation that you are.
Using software from the past requires that the content you create also stays in the past. While you may be able to open an old file in the newer versions, it will often lack editability. You run into the same issue trying to share files between different vendor apps.
I'm assuming you are looking for a cheap way to do something with the software that you already have. If you don't know how to use the software now, learning different software that does the same thing is probably the better way to go. For one, the interface has changed significantly between then and now, so you gain nothing by learning old software with the intent of applying said knowledge to the new. There are a lot of open-source (free) applications out there that do a pretty darn solid equivalent job and are modern as well. Best to apply your learning curve there than with older software. You will likely find that today's hardware is less receptive to yesteryears software. Video and audio codecs being a good example of this. Not to mention the hardware limitations imposed by the older software.
