The free Sonar Herb mentioned has a GREAT built in drummer called Session Drummer that you can as Herb says "mouse your way through"--that is play the virtual kit with a mouse-- to create realistic fills and even secondary tracks (or main tracks if you use no Real Drums.)

I use it all the time and it is super easy. You load a style and kit--like Old Zep Wet Kit--choose what type of fills or patterns you want, then just click on those while the song is playing, recording to a track.

Mixed in with Real Track you will never notice the paste if you EQ them the same.

I think it took all of a half an hour to learn. It really is not that hard at all.