The answer is YES. You've got a Vista OS now, you can install another Vista
on a different partition. I will be very easy. it typically takes close to
40 min or less. Vista takes care of all drivers. You don't have to worry
about anything. It is good to have another partition clean but if it is not,
Vista will take care of it also. All your important file swill be collected
to Windows.old folder.
Your second Vista will become the primary Vista so to speak. It will be the
default OS by default unless you changed it in CMOS.
It is not a sound idea on average to have two OSs on one machine unless you
have special needs. I cannot even thing of a situation that will require it.
What happens is you will have time barely for one OS and the other one will
fall in disrepair, no updates will be installed. When you sign in there
perhaps once in a quarter the other Vista will have a very hard time to
bring you up to date.
I think the existence of two OSs on one machine may be useful only in cases
when your default developed problems so sensitive that you cannot do repairs
even in safe mode. Then you go to the other OS, endure all updates,
sometimes close to 15 (a real case for one of my XP partitions) and from
there you can change any dll or system driver without being punished.
I do have two partitions on 3 machines but it happened historically. I do
use some of them in dual role.