Ok, I already had The Witcher downloaded and was not bound to download the game again for a 16gb file. Also, as many of you know by now, you cannot install the upgrade patch from the official website as they ask for a registration, which requires a CD-Key. So I tried this on my multi language version of The Witcher v1.0 and it worked perfectly. Just follow the instructions step-by-step and hopefully it will work for you too.
1. Download these files for your chosen language format from http://www.thewitcher.com/community/en/www/installation_of_twee_in_four_easy_steps.html
2. Now install your original version of The Witcher.
3. After the installation has finished, replace "witcher.exe" file located in "C:\Program Files\The Witcher\System" with the provided rar file "1045_witcher.v22.214.171.1242.nodvd.rar". This file can also be downloaded from http://gamecopyworld.com
4. Now double click on "TWEE_Upgrade.exe" and install the update. You\'ll notice that it bypassed the registration message.
5. Once the upgrade is done, open "TheWitcher12.7z" and copy that "witcher.exe" over the one in "C:\Program Files\The Witcher\System" for the last time.
Note: This file is a witcher v1.2 RUS patch which can also be downloaded from gamecopyworld, but hey, it works.
6. Now start the game and enjoy The Witcher: Enhanced Edition!
I know it seems confusing by replacing the EXE file so many times, but follow the instructions carefully. Once again, I tried this with a Multi5 iso from PLATiN TEAM which was released on 10-24-2007, and downloaded all updates with the English language. I hope this works for you as it did for me!