I currently have Carrara 4 standard. I will be getting the upgrade to C5. I have never bought a software upgrade before. Do I install the upgrade over the current version, or do I have to re-install the whole program?

The installer wants to check if you deserve an 'upgrade', most of the time it will search your HD for the previous version, if it is not found it asks for the serial number of the program you want to upgrade from. Most installers are this flexible, I don't know what the Carrara installer does when the 'old' version is not on your hard disc, because I upgraded (since RayDream 3 or something) while the previous version was there. Most software upgrades are installed as a 'new' version, so you have to transfer your plugins to the appropriate directory to have them working again. With 'updaters' the existing application is modified.