InteractCD is now closed. They never sold games for download - just physical disks.
Any download site you buy a game from has a procedure that is stated in the information or FAQ in their site. Some procedure varies from site to site.
Some have direct download; some require that you download their "downloader" or "control panel" or their specific program (like Steam or Amazon).
So it would be best to check the download store you want to buy from and see which procedure they follow.
I will also move you post to the Discussion forum.