Don't worry. This is easy, and you do not need to make another purchase.
If you ordered a physical DVD-ROM, just use that. If you got a new computer without a disc drive, that is not a problem: email us with either your original order number or just include the serial number from the box, and we'll send you a download link for the version you purchased.
If you order a download, the setup files will be left in your Downloads folder after installation is completed. You should probably take a moment to make a backup of those setup files (copy them to a memory stick or external drive). If time goes by, and you no longer have your setup files for whatever reason, we will be happy to re-send them to you. Please just send a support email requesting a replacement. Include the full name and email address that was used to make your original order, at the very least – the actual order number is even better. If you can provide the full name and email from your original order, you'll get a replacement very quickly. If you send a vague email with no name and an email address you did not use before, things might take a tad longer, so start out by giving us as much information as possible so we can verify your order and get your replacement setup files to you promptly.
The bottom line is: you do NOT need to make another purchase to re-install. However, if it's been a while, you may want to consider upgrading.
The evaluation version from this website is not what you purchased. The free download from the download page does not include the library you paid for, so you will want to contact us for the full version if you are re-installing and lost your setup files.