That's actually why I asked. Believe it or not it's pretty common for people to rhyme an acapella and then try to find a beat to match it or even to listen to a beat in their headphones while they record and try to match it up to the beat afterwards. 99 times out of a 100 though it ends up sounding off.
Try to get your hands on a recording program with multiple tracks. Adobe has one call Audition I think. That way you can play your instrumental in one track while you record to another.