Where is %systemdrive% defined?

Hi group,

There was a thread about this back on 10/22, but the OP didn't follow up
until today, so it may have been lost. I've been wondering this same thing,
and that is that if I look in the registry, I can see where some environment
variables are stored (HKLM\system\current.\control\Session
Manager\Environment. But, some vars, such as %systemdrive% are not stored
there and do not seem to be stored anywhere in the registry. So, my guess
is that these vars are loaded defined and loaded into memory each time the
OS loads. I know ~what~ %systemdrive% is, but I'd just like to verify that
this variable is NOT hard-defined somewhere and is just something that's
loaded into memory.

The main reason for this is that if I want to get the %systemdrive% off of a
remote computer, what hard-defined var can I get it from 100% of the time,
not 99.999%?

Thanks,

Ray at work