Multiple SQL 2005 Express installations can coexist on the same machine along with other installations of SQL Server 2000 and SQL Server 2005. SQL Server 2000 instances must be upgraded to Service Pack 4. The maximum limit to the number of SQL Server Express instances is 16 on the same machine. These instances must be uniquely named.
SQL Server Express by default installs as a named instance called SQLEXPRESS. This particular instance may be shared among multiple applications and application vendors. Microsoft recommends that you use this instance unless your application has special configuration needs.
Please see SQL documentation for more details.
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