How do digital downloads work...?

I bought serious sam gold off of a direct2drive like website. I downloaded the game, extracted and inputed the key that came with my purchase to unlock the executable. Works great. I wanted to see if I could put the game on another computer so that I could play coop with my little cousin. So I installed the download file on the other computer. I inputed the same key to unlock the executable. When I tried to run the game it gave me an error saying that the dvd was not in the dvd-rom drive. I'd like to understand why this particular error comes out. Was the error purposely installed to prevent people who bought the game to not distribute? Is it a problem with my computer or file? I'd like to understand how digital downloads work when installing on one box and another.

Sounds like copy protection to me. It makes sense if you think about it though, you paid for one copy, not two.

A no-cd patch might work. Try here…

http://search.gamecopyworld.com:9999/data/gcw.shtml?search=serious+sam

GoG is the only way to go with the older games. No DRM of any kind.

What does that mean 'D2D-like'? I know od SS Gold at D2D, Amazon digital and GamersGate. GG and D2D should work on multiple PC's, but don't know about amazon. And most games with a code will prevent you from sharing the game on a network.

I got SS Gold at D2D and it had a code. Going to try the no-cd crack.