I keep getting this error when I try to install it:
(See the attached picture)

Any help would be greatly appreciated. Thanks in advance.

I don't see any errors in that picture. The box is asking you whether or not you want to stop the installation. Did you try to stop it before it was finished?

I would click "No" and let it continue.

I just completed a playthrough with the latest RotWW, and it worked fine. I had no problems, but I'll help if I can.

That is the error, it gets interrupted, ALWYS.
After I click Install I get that; if I chose "no" it sends me to the start (license agreement) and if I chose "yes" and cancels it.

I understand.

I would try installing RotWW over a clean Witcher 1 installation, if you haven't done that.

WITH or WITHOUT The Witcher installed is the same behavior/problem.
The installer simply refuses to work on my system (I never had nor currently have that issue with installers).

I'm sorry to hear it. It's a very good mod.

I wish I had a good way to fix this, but I don't know what the problem might be. You might try contacting the creators.

I have a thread over forums.cdprojektred.
Cheers for the replies.

I would try all the usual suspects...

Are you running it as Administrator? It could be a permissions thing.

Is the base game installed in C:/Program Files? That tends to cause problems.

Do you have the newest version of Thw Witcher 1? The RotWW instructions say you have to have the latest version.