Allanc,

Yes, you can have both on Windows 2003 server.

However, tools issues mean that it is best to install SQL Server 2000 first
(even if it goes into a named instance), then install SQL Server 2005
afterward.  For compatability reasons, SQL Server 2000 should be at SP3 or
SP4.

But your SQL Server 2005 is already installed. Do you have a test machine to
try this on?  If so, you could go through the paces a couple of times and
work out the problems.

I have not had to do this, but to play it safe I would suggest that you
should:
1. Backup the SQL Server 2005 databases, then detach the user databases.
2. Uninstall SQL Server 2005.
3. Install SQL Server 2000 as a named instance.
4. Reinstall SQL Server 2005 as it was before (either the default instance
or a named instance).
5. Restore master and msdb from backup for the SQL Server 2005 instance.
6. Reattach the user databases to SQL Server 2005.

FWIW,
RLF