OVERVIEW
You need a DAW (Digital Audio Workstation), which is the main program. The DAW runs (or "hosts") instruments and effects known as VST (Virtual Studio Technology) modules.
DAW
A DAW can be anything from full-blown professional studio software to simple programs that host a single VST. The professionals use highly-technical commercial software like Cubase, Fruity Loops, or Reaper, and those bad boys will set you back a couple of hundred bucks (or more) a piece. There is an open-source option, LMMS, which has some fans. These DAWs are amazing for producing, but they can be dauntingly complicated.
I use Cantabile Lite. It is free, does everything I need it to, and is simplicity itself. It's designed for live work: jamming, practise, and gigging. It's probably the easiest way to turn your MIDI controller & PC into an emulation of a vintage keyboard, rather than an emulation of a full-fledged recording studio.
Cantabile works on virtual "racks", just like studio equipment. You place one (or more) instrument VSTi modules into a rack, then (optionally) some effects VSTs, and then you play music! I won't write a tutorial for Cantabile, it's too easy to use for that, but if you're struggling there are some free tutorial videos.
VST
The most important software are the VSTi (instrument) and VST (effect) modules, such as a Hammond Organ B3 VSTi, and Leslie Speaker VST modules. The site does not provide a comprehensive list of all Freeware VSTs, only the ones I personally consider the best. The LINKS page provides you with other larger sites that will let you hunt down and experiment with other VSTs.
DRUM MACHINE
Cantabile comes with a metronome, but I don't like a sterile click to practise to. I use Rhythmus, a simple Freeware drum-machine VST module that emulates the "classic" Korg Mini Pops drum machine. (As used by the famous Jean Michel Jarre in Oxygène). You can mix and play different beats at different tempos. It doesn't do complicated, programmable time signatures, but it does enough to get me jamming away.
AUDIO DRIVERS
Finally, a word on audio drivers. The audio drivers that come with Windows (Direct X) work for games, but they are not that great for music for various tech reasons I won't frizz your afro with. The standard, high-quality alternative is called ASIO (Audio Stream Input Output) and you can download ASIO drivers at this site. The drivers won't interfere with your existing system, just gives you an extra option in your DAW. Trust me on this, brothers and sisters.
Peace, Funk, and Funky Peace!