Only allow one session per user

I require coding that will restrict users logging in to an ASP application only once. Currently when a user logs in I validate their login and password against the database, then setup the session variables. However, if the user goes to a second computer they can login again whilst they are still logged into the first computer. I need to be able to stop this from happening. However, if the user logs into the first computer and opens a second browser they should still be allowed to continue working with both browsers open on the same application. This is to allow them to perform two functions at once. For instance if they receive a phone call when in the middle of working on something they should be able to open a browser, do what they need to for the phone call then shut down the second browser and continue working with the first. I have a logoff link for when the user is finished working which currently removes all session variables, however not all users actually logoff. Some just use the 'X' at the top of the browser or if their computer crashes, they may just need to re-boot. I am open to suggestions on how this is to be achieved. Please provide your proposed solution and the timeframe it will take you to code your solution. ## Deliverables 1) Complete and fully-functional working program(s) in executable form as well as complete source code of all work done. 2) Installation package that will install the software (in ready-to-run condition) on the platform(s) specified in this bid request. 3) Complete ownership and distribution copyrights to all work purchased. ## Platform Windows Server 2000 ## Deadline information ***Code must be well Commented***