When you're working with a PC you should always be aware that a situation may occur that causes you to loose all the data on the PC. This is not rare it happens a lot and the chances that it will happen to you in the next 10 years are higher than 50 percent. Therefor you must make backups of all the important files for which you don't have a backup yet .
When you buy a software product with electronic delivery that means that you won't receive a CD-rom that can be used for backup. Instead you should make a backup on CD-rom or USB-memory-stick or portable-disk-drive. This means making a copy of the file you downloaded and the confirmation email . The registration key is included in the confirmation email that also included the download information. You are asked for the registration key the first time you start the software.
If you won't take the trouble to make a backup then that is your choice, but that means that you will have to pay again if you loose the file and/or the registration key.