Let them know that you are very interested in working for the company as you have always admired it and like it's model or it's reputation or whatever it is that you like. Tell them that there are several positions that interest you, but the one you are interviewing for seems most like what you would like to do. If you interview well, they will probably want to know what other positions you are interested in. Hiring managers are always talking to each other and will all likely be in a meeting discussing viable candidates for these positions (unless you are in a very low level position (I am not trying to be a smart ass)).
I once did not get a job and the hiring manager said, "We have gone another route for this position, but we are very interested in having you work for us. We will be in touch if something should open up like I think it will." Three weeks later I interviewed for another position with a different manager and took the job within the week of interviewing. The guy chartered a flight to meet and interview me in Atchison, which is like the worst attempt at a wackycatbrag that I can imagine. I wasn't living in Atchison, but it wasn't too far away. It was an easier airport for him to land in.