MD5 error when trying to install GalCiv II

Hello, 

On Feb.06 I purchased GalCiv II through impulse. I have a fairly slow DSL connection, so the installation took about 12 hours. At the end of it I received the dreaded 'MD5 invalid' message. 

I'm running Windows 7 64-bit.

I searched for help and found someone with a similar problem:

https://forums.galciv2.com/363625

I followed the suggestions in there, and ran memtest and did a full-system Avast scan. Both passed without any errors/infected files. I disabled all real-time protection and tried again. About 12 hours later: same result. MD5 error. 

Last night I dragged out my old Windows Home 32-bit laptop and set it up, and tried to get it to do the 'download and archive' on GalCiv II. . No firewall/protection running, reset the router, disabled any security feature I could find, and tried again. I woke up this morning to the same MD5 error. 

Where do I go from here?