Find which user folder the current Outlook.pst file is in. Copy that file to a different location and then replace it with the Outlook.pst you backed up a while back. You should now have all your emails that were present when you did that original backup. As already mentioned, depending on the Outlook version, you will only end up with your outlook folder structure and your emails. You will have to set up your accounts once again and possibly the rules if you had any. From memory with Outlook 2003 it kept the account settings but you lost the rules but with 2007 you lose the account settings too.
You will lose all emails between the date of that original backup and now but then if you want to get them back you will need to replace Outlook.pst with your latest one. I'm not sure if there is any way of merging pst files so that you get all the old emails plus your recent ones so surprise me if you find a way!