CS4 Master Collection trial installation blocks CD installation

Question

Hi!
I'm  need someone to help me. Here is a description of what happened, when I installed the Creative Suite (CS4) trial and tried to install the full version:
1) I installed the trial version of CS4 Master Collection from a CD because I didn't need the whole version (or so I thought at that point, I was stupid as seen later).
2) The installation screen told me I was able to insert the activation key later if I wanted. My impression was that I can do the installation and then upgrade to the full version if I wish.
3) I run through the installation smoothly. No errors occurred during the installation.
4) I could use the trial for 30 days after which the version expired.
5) Then I tried to upgrade to the full version using the CD activation key. Unfortunately, the key didn't work but rather showed a red cross and an error message popped up saying the activation key was "not valid".
6) My friend told me I should remove the trial version and then reinstall from the CD. However, the same problem also occurred in some other computers, so I believe that's not the only cause of the problem.
7) After spending some time on internet, I found the following blog dealing with the topic:
http://sherylcanter.com/wordpress/2009/02/problems-removing-and-installing-adobe-cs4/
I read some of the messages posted in the blog. I started searching for the cleaning tool  mentioned in the blog, and finally found the tool from Adobe web:
http://www.adobe.com/support/contact/cscleanertool.html
I would really appreciate if someone with similar experience would tell, whether this tool really works in the latest versions of Windows (Vista, 7 & 8, I have 7).
I would also appreciate any assistance on the topic.
I already tested removing the "cache"-file from /C:/Program FIles/Common FIles/Adobe/Adobe PMC/cache/.. My effort had no effect.
If I really need to install my system again, will I lose all the files on the computer I'm using? I don't want to believe  that. Probably I can store some of the most important files on an USB-memory and still be able to use the Suite when it's installed and the files returned to my computer? I surely need to backup the system on my PC and not the others that were connected to it (any additional backup would be a catastrophe )?
I'm sorry for all the stupid questions & stuff, but I'm trying to get my message as clear as possible.
CS4 is an expensive software, so I really need to get it work. Installing the Suite from a CD should be possible because anyone may have  a trial on his PC before buying the full version on CD. That's the purpose of the trial version, I see.
Thank you for any help!

Answer 1

Thank you for a fast reply!
The problem is stated in the title of my previous message: I cannot install the full version because I installed the trial.
Formatted as a question: "How can I get the CD installed with the original serial number?"
You wrote: "And where did you actually buy your CS4 license? There's a lot of fakes out there..."
>The license belongs to my employee. I'm totally sure that the version I'm trying to install is not a fake.
"Other than that you could simply have used a wrong region installer for your trial and the serial number ranges are not compatible"
>Could you clarify that more. The Suite has worked perfectly before I installed the trial. The installer only asked me to accept the license agreement and to choose between the trial and the full install. I was not asked anything else.
"You lose yourself in lengthy descriptions without actualyl ever telling what the problem really is, what the exact text of the warnings/ error messages is, what exact system you are on and so on"
>I admit my "question" was lenghty and somewhat messy. Here are some more details:
Country: Finland
Operating system: Windows 7 (as mentioned earlier)
Computer: Fujitsu (all requirements of the Suite are fulfilled, as the trial worked well)
Exact error message: "The serial number you entered is not valid. Click OK to retry entering the serial number."
Situation: I installed the trial, waited 30 days, and the trial expired. After that I tried to insert the disk serial number.
Possible solutions: Removing the trial and installing from beginning.
Internet access: Was ON during the installation.
The program has installed correctly earlier. Even with the serial number put in the back side of the disk.
Right now I don't have the information of the place where the Suite was bought (probably the origin is not in Finland). I can find it out on Monday. Possibly it was bought used that would make tracking a bit difficult.
Please ask if anything is still unclear. I know more on Monday, so if you can specify what information is essential by then, I would appreciate, and come on Monday with the asked pieces of information.
Hope this message clarified something.