Some website keep old programmes for downloading, because people still use and like the older versions, as an example IE6/IE7.
If you download and use an older version programme it may have a block for future updates, because the company regards that particular product as obsolete and perhaps by its sell by date. Some companies continue to provide updates for older versions, but these usually reduce in frequency, so in the case of an anti-virus product, you safeguard and protection can be limited, and that product is best removed from the computer.
If you keep getting pop-up reminders, then that is what the product supplier wants you to do, update. Whether you stick with the product or move elsewhere, then thats your choice. I run ZA (free version) and keep getting reminders about an update version availability. At present I keep pressing the 'remind me later' button, this is to see if anyone responds via a forum like PCA of any problems due to downloading the new version.