If you search around the forum there are a bunch of post with the answers you want.
Anyway downloading in different parts mean that the files of the game are compressed and uploaded to hosters in different parts, so you need to download all parts, and when you have all then you need to extract the content to your SteamApps folder (you need Steam installed and Cracked) after you put the GCFs in that folder and run your Cracked Steam then you will see the game in your Games List Tab, then just run it, no instalation procces is needed.
Edit: Oh wait, i think Dead Space is NCF not GCF, so what do you need to do is put the NCFs files en SteamApps folder and the Dead Space folder in SteamApps/Common.