the easiest way is to get MSVC, open the workspace and hit F7
It should compile right away. (ofcourse you need to set up your libs, hl sdk dirs, but i thought the 'original' HPB could compile without these.?)
you don't compile bot.cpp alone, you compile the entire project. Bot.cpp is a part of the 'bot'. A bot needs more then just one file. The project exists out of 'parts' , ie, a bot brain. Access to the DLL, hooking, etc. Most stuff you don't really need to bother around the first time.
When you get it compiled, you should worry about bot.cpp , later on you can figure out how the other stuff works. Eventually , just follow the flow of the functions and their calls.