为什么从SQL Server 2000升级到2005会导致查询速度变慢？

Are there any common reasons why upgrading a database from SQL Server 2000 to SQL Server 2005 would result in slower queries? This is coming from an ASP.NET 1.1 application with hundred of tables, everything is indexed and seems to run well on the older version.Are there any common reasons why upgrading a da