They are the different instances of SQL Server. You either have multiple instances running or have installed, uninstalled, then reinstalled.

>I noticed that on my PC where I have SQL Server 2000 there are folders MSSQL and MSSQL.1. On the PC with SQL Server 2005 there are folders MSSQL.1 and MSSQL.2. All of these folders have DATA folder.
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>What is the significance of having these different folders? I am moving a database from SQL 2000 to 2005. Does it matter if I put it in MSSQL.1 or MSSQL.2?