Hmmm never seen this before, you sure your computer isn't blocking something (a firewall or whatnot?) integral?

I'm not a moderator, or any expert on the game, or programming for that matter, but when you say you're "downloading with both firewalls off and the virus scanner turned off," do you mean that you're actually downloading? Or installing? I don't really see firewalls or a virus scanner interfering with downloading...
Maybe I'm wrong, just thought I'd input....

Yeah, I was talking about actual installing... though theoretically the firewall could interfere with the download if it perceives part of the package as a virus... are you using a mirror, or this site... and does it make a difference?

i just tried myself and downloaded FO3PR from our server and it executed fine for me. so from the server side, everything seems to be alright. there is actually no way to get a corrupted installer to work as the installer IS corrupted .
while unlikely, viruses can cause the installer to get corrupted by modifying the exe file. alternatively you could try to change the way you download, if you download through your browser, try to use a download manager. if you already do use a download manager, try another one or change its settings to be less aggressive. some download managers use the servers resume function to split downloads in many parts to gain a higher download speed under some circumstances, this can cause corruption also.

btw welcome to the forums