A worthy goal. Learning a language over the last 5 years has been a very rewarding and consuming experience for me. Much harder than I had expected it would be, but really worthwhile.
I tried to learn Russian and then Mandarin through Rosetta Stone (our office also has a program, do at home/own time) but I was woefully bad at both. I enjoyed the program though.
Spanish seems like it would be easier, and more useful.
I have found through the years that I am pretty good at picking up languages, so I think Rosetta Stone will be good. Of course, no one has answered my email yet, so I'm going to have to hunt her down.