installing postgres7.3.4 problem

Hi all.
(I'm a newby. I recognize it)
At the moment I had postgres running on my Linux machine with no problem.
I had version 7.2 installed, and this version came along with my Mandrake
distribution.
Now, I want to install a contribution package (postgis extensions), and
previously I would like to install a newer postgres version.
So, I've downloaded the postgres-7.3.4 source code and compiled it. But,
following the guidelines in the documentation, after "make install", I've
typed:
# su - postgres
(I don't know the meaning of a "login shell", but this is not my problem)
And then appears the message (in spanish):
"su: atention: we can't change to the directory /var/lib/pgsql: the directory
or file doesn't exists"

Previously I had postgres databases in /var/lib/pgsql. But I've uninstalled
the previously the 7.2 version and I don't know why this happens.

Could anyone guide me with this?

thanks and regards

Javier