The following side-by-side conditions are not supported in this release:
- Side-by-side Tools - Presence of SQL Server 2005 Management Tools or BI Development Studio will block installation of SQL Server "Katmai" Management Tools and BI Development Studio.
- Side-by-side with SQL Server 2000 - An instance of SQL Server 2000, including MSDE, will block installation of this release of SQL Server "Katmai".
- Side-by-side with SQL Server "Katmai" CTP1 - An instance of SQL Server "Katmai" CTP1 will block installation of this release of SQL Server "Katmai".
- Side-by-side instances of SQL Server "Katmai" CTP3 - Multiple instances of this release of SQL Server "Katmai" are not supported on a single computer.
This is from BOL of SQL 2008.