Unlike other Microsoft products, it's not possible to just click "Help/About" to get the version and edition information of your SQL Server installation. I was curious about mine and searched around. I found this MSDN article that describes how it's done for each version of SQL Server. For SQL Server 2005 the query syntax is (courtesy of MSDN):
SELECT SERVERPROPERTY('productversion'), SERVERPROPERTY ('productlevel'), SERVERPROPERTY ('edition')
Or you could just download and execute my query file from the SQL Management Studio.