Waltorious: Did you INSTALL the downloader as admin, as well as run it as admin? If not, try that.
Can you download large files without corruption from other sites? With my old computer I had an issue where downloading or unpacking large files would always fail, no matter where they were from, and it turned out to be a problem with my RAM. Hopefully that's not your problem!
You could also try contacting GOG support to see if they can help. The fact that the downloader never even starts is weird.
Running it as an admin still doesn't work.
A week or two ago I downloaded and installed The Witcher without any problems and that game is a few gigabytes bigger than Two Worlds. I contacted GOG support now I'm just waiting on a reply.