changing user id

Hi
I'm a bit of a noob with PostgreSQL and I hope this is the right list to ask this question...

When postgresql loads there is and error message:

postgres@XXXXX:~$ /etc/init.d/postgresql-7.4 start
chmod: changing permissions of `/var/run/postgresql': Operation not permitted
 * Starting PostgreSQL 7.4 database server:main
Error: Could not change user id                                     [fail]

The error message in start-up script /usr/share/postgresql-common/PgCommon.pm
Using Linux-Ubutu.

Reading the script suggests that the user to be a member of the "shadow" group, which is now done.
However, postgresql is still not starting and produces the same errors:

Are there special privileges that the "shadow" group has?

What should also be done to getting  postgresql working again?

TIA

Russell