I use software called Cakewalk. It's pretty much a virtual recording studio. It's fairly easy; the only con is that you have to convert your tracks to wav files.
But the nifty thing about it, is that you can import both the original and instrumental tracks, and choose to record only on the instrumental. Both of them will play but it'll only record on the track you want it to. So if you're using headphones, you can use the original track as a guide for pitch and timing.
I also like it because you can do multiple takes, splitting up the parts instead of doing it in one go. And if you mess up you can just undo the most recent recording and try again.
I've also heard people who use Audacity and Goldwave... I don't know about those but just some other suggestions I can think of, off the top of my head.