Normally you should only need to enter your serial number the first time you launch ArtRage. However, if it happens repeatedly, this means that something is preventing ArtRage from storing its serial number information.
Launching ArtRage as an administrator should correct this. Browse to your version ArtRage in the start menu and right click it, then choose ‘Run as Administrator’ from the menu. On Windows 8 or later you may need to select ‘More’, before selecting ‘Run as Administrator’.
ArtRage should then launch and ask for your serial number. Enter it, click OK then quit ArtRage and start it again. It should now start without asking for your serial number.