3DRealms and Human Head Studios have released the Prey SDK (Software Development Kit) which includes all of the game code (for building your own Game DLL), as well as some code examples/tutorials which will explain a few of the basic things you should know before diving into the code-base. This SDK distribution also compiles without issue in the free version of MSVC++ 2005 (Express Edition).

Prey SDK (3.91MB)