Win2K Questions

A company I am currently doing work for has MS SQL Server 2000 running on a
dedicated host. In the near future hosting of the website and other services
is going to be brought internal. With that they face a very large cost for
purchasing SQL Server and the various licenses. I heard a few people mention
PostgreSQL as a possible solution, and at the outset it seems to support
many of the things that would be required, however, I do have a couple
questions.

How does the performance of PostgreSQL stack up to SQL Server 2000 or
Oracle? Is it even aimed at being an enterprise level database?

What is VACUUM? I have seen this mentioned in various places, saying that it
needs to be run at times. What is it exactly, why does it need to be run,
and is there any way to automate it's running?

Lastly, where can I download an executable for Win2K so I can go ahead and
take it for a spin? I already found the pgAdmin app for administration
purposes, so I just need the database engine itself. Also, any tips for
installation on 2K would be most appreciated.

Thanks,
Steve