Multiple instances with same version?

Dear Everybody!

I want to ask that have some way to install PGSQL 9.0 as two instances
in one machine?

Most important question. The OS is can be Windows or Linux.

I asked this, because formerly we heard about a story. I cite this as I
remember:

We have some product, and in the only one server of the customer
(Win) have a PG9.0 version installed.
But the developer company lost in the space, only the software
(must) working...

We don't know the root password, and we don't want to hack it (the
system must work).
But we needed to install the our version of the PG what is also 9.0
(because of the new functions)...
...

We want to prepare to same situations with learn about PG.

With Firebird and MS-SQL this case is not problem, because on same
machine we can install another instances with same version.

But I don't know that is PG supports multiple instances with same
version or not?

Also interesting question are the "users".

In our systems we create user for every real user. If they are 100, we
have same number of db users.

But if we want to server more database in one place, we may do conflict
on users. For example: all of the databases have user JohnM.

If we can do multiple instances, the problem is vanishing, because all
have it's own user list.

If we cannot, then only idea if have if we make prefix on usernames
based on short dbname.
For example:
offer_db users: off_JohnM, off_MaryK
press_db users: prs_JohnM, prs_TomR

Please help just a little to I can know the limitations of PG.

Thanks for it!

Regards:
dd