Hey Klanctot,
Dale is better at this than me but I'll give it a shot.
Since you are seeing the "You entered an upgrade registration code in the original registration code field" I would guess that you are unistalling 3.321 before you upgrade.
Make sure 3.321 on your machine and try to install 4.08. If you purchased the upgrade (which the error message says you did
) it should install.
I hope this helps.
Jim