you've been able to do a full, clean install of Windows with an upgrade disk as long as I've been using a computer ;o).

What it's telling you is that you need to do a clean install for the 64 bit. You can't just upgrade if you'e going from 32 bit to 64, so follow what the disk is telling you - reboot with the disk in and do a clean install. Back up things you want to keep, because you'll have to reinstall the programs you use after you've completed the install.
Laurie