I've heard Rosetta Stone is GREAT, but that's mostly been from their own promotions. I don't personally know anyone who has used it. It's rather expensive too; I don't know what budget you want to work within. You can test it out on their site to get a feel for how it operates:
Rosetta Stone: Learn English, Learn Spanish, Learn French, Learn German, Learn Italian, and 25 other languages with our language-learning software packages.
If you want to get something for free, go to your local library and check out language lessons there - they should have various programs, tapes, books, CD-ROMS, maybe even DVDs. If you find one you like you can always buy it later.
Another thing I've seen is a game for the Nintendo DS called "My Spanish Coach", but again, I don't know anyone who has used it, so I can't vouch for it's effectiveness. You also must have a DS....

Here's a link with some user reviews:
Amazon.com: My Spanish Coach: Video Games