I suppose the first question I must ask is whether you have permission.
Most forum software is downloaded and just has to be installed to work, like software from phpBB.com. If you know what forum software is used, download it and set up a framework the same as the one you want to copy. You will have to set up the forum categories and various user permissions and registration procedures to be the same as the one you want to copy.
Then, if you have permission, you could copy the posts from the backup files of the original forum. The administrator of the original forum could email this backup file to you. Or if you have a username and password on the original forum and the administrator upgrades you to an administrator you could do this yourself.
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.