Lazi,
I'm not an expert on the subject, but I've been working with acapellas nonstop for the last few weeks so here's some things you might want to try.
First, I would advise you to get ACID if you don't already have it. Even the free version could be used, although it obviously will lack some features. You could just as well use Sonar if you're more comfortable with that (I'm not sure on it's looping features though).
1. If you have ACID you could just use the BeatMapper function, although it's not always accurate depending on what info you give it. If not then....
2. Just import the acapella into the program and then get a very simple 2 or 4 bar drum loop (1 and 3 on the kick, 2 and 4 on the snare and straight 1/8s on the hi-hat). Now you're going to have to find a downbeat (1 as in 1,2,3,4)in the acapella. Easiest place is usually at the beginning of the hook. Line your loop up with the downbeat and then play with the tempo until it feels right.
3. If you have the original version of the song you could just create a loop (4 or 8 bar) of the original song in a wav editor. Import the loop into a sampler and play it along side a simple drum loop from your sequencer and play with the tempo until they match up.
Just some ideas for you to try, hope this helps.
Stray