If you are called for a second interview this means that you have 90% chances to get the job.At a second interview be ready to discuss about the salary, the schedule, you will meet some of your new colleaguesand perhaps you will have the opportunity to see your office. :)
See this site for more info about second interviews: http://www.cvtips.com/second_interview.html
And of course you don;t have a 90% chance of getting the job - if they invite 2 people back for a second interview then surely you have a 50% chance? Less if there are more people invited back!