Often software will request a certain service pack level or version of SQL Server. (IE: MOSS 2007 requires SQL 2005 SP2). There is no simple Help > About to see the version of SQL you are running.

For SQL Server 2000 and up, run the below query (2000 - open Query Analyzer, 2005 - right-click a DB and select New Query):

SELECT 'SQL Server '
+ CAST(SERVERPROPERTY('productversion') AS VARCHAR) + ' - '
+ CAST(SERVERPROPERTY('productlevel') AS VARCHAR) + ' ('
+ CAST(SERVERPROPERTY('edition') AS VARCHAR) + ')'

This will return your version. You can see the build charts for more detailed information:

SQL Server 2000 Build Chart
SQL Server 2005 Build Chart