As explained in many other forum threads about the missing python25.dll, the only possible reason for this error, is that you are NOT running the latest Couatl.exe, but the previous version ( v 2.0.x ).
The python25.dll was required ONLY by Couatl 2.x. The latest version DOES NOT requires it. It requires Python27.zip/dll, which are included in the current installers.
Since the new installers now (correctly) REMOVE the un-needed Python25.dll, if you are stuck with the OLD Couatl.exe for some reason, you will see an error message about the python25.dll missing. Of course it's missing, it has been removed because it's not needed anymore by the new version.
The latest Couatl.exe ( 3.0.x ) is included in ALL our *current* installers, and it's not available anywhere on our site so, using the current installer, it's NOT possible to get the old Couatl.exe by mistake using our current installers.
The only possible reasons for not being able to install the latest Couatl.exe are:
1) You are NOT using the latest installers. Maybe you downloaded them, but you are not installing them. I experienced this first-hand on a user's system when doing remote assistance: he downloaded the latest installers before my eyes, but then launched an older version, because it went to the wrong "Downloads" folder, not the one the browser was really downloading into...
OR
2) Your antivirus/firewall is BLOCKING the installation of the Couatl.exe, so you are stuck with the old one but, since the installer correctly removed the old .DLLs, you will see the error, because you those .DLLs were needed by Couatl 2
As explained so many times on the forum, it's best turning off the antivirus as the first thing to do when something that looks wrong happens with the installation.
You said you have correctly configured the antivirus, and you said the fsdreamteam folder was empty so, in your case, the only possible explanation is #1: you are using OUTDATED installers.
Yes, I see you say you have use them, but it's just not possible you would see this error if you had. Check your downloads better, and be sure you are installing the .exe you downloaded, from the actual folder you browser *currently* downloads into.