I would disagree with the fruity loops suggestion.

I would say use a vsti sampler with MIDI functions and record your riffs to tempos and what not. The problem with using fruity loops and preprogramming your drums is that you cannot change things as you go unless you reopen the program and re-import the files.

With MIDI you can change as you go, record guitars first then put drums or vice versa.

You could download LoopAZoid for free as a drum sampler, that's what I use. Or you could get a good program and use DFH. I have only used Cubase LE but it works just fine for me.