A 'clean' install is when you wipe the hard drive clean before installing the OS. You cannot "upgrade' from 32 bit to 64 bit because the kernel of the two operating systems is completely different.
You should be able to do a full and clean install from your 64 bit CD. Use the licence key you have. If Vista doesnt' like the key, it will prompt you to call Microsoft - Do So, and tell the operator you are switching form 32 to 64 bit. They'll give you a new key on the spot.http://www.vistax64.com/tutorials/117366-clean-install-full-version-vista.html