Can I install SQL Server 2000 and 6.5 on the same machine?

Yes, you can install SQL Server 2000 and 6.5 on the same machine if SQL Server 2000 is using named-instance support. In this configuration, you run SQL Server 2000 as one (or more) named instances and SQL Server 6.5 as a default instance. However, we have a couple of caveats about this setup. Avoid running SQL Server 2000 and 6.5 together in clustered mode because the technology used to cluster SQL Server 2000 is incompatible with previous versions. Save time by going to http://support.microsoft.com and reviewing Section 6 (SQL Server 2000 install orders) and Section 22 (how SQL Server 2000 named and default instances work side by side with SQL Server 6.5) of the Microsoft article "Frequently Asked Questions—SQL Server 2000 Setup" (Q257716). "Basic Guidelines for Installing SQL Server 6.5 or 7.0" (Q192710) focuses on install orders and explains the pitfalls of running in clustered mode. Also read "Supported Windows Versions for SQL Server" (Q122352), which warns that when you install the SQL Server 6.5 server components or client tools on Windows 2000, you might receive the following error message:

<i>This program does not run correctly
on this version of Windows. To continue,
click Run program. To exit, click Cancel.
For more information, click Details.</i>

You can safely ignore this message. Click Run to continue with the install.

Another caveat: When you run scripts with administrator permissions, be sure that you're connected to the correct instance. Confirming an accurate connection will help you avoid unexpected errors such as running obsolete commands against SQL Server 2000 or SQL Server 2000-specific commands against your SQL Server 6.5—and an unnecessary phone call from your operations team.