Тут же хочу обратить внимание на полезный пост из ранее прорекламировано блога.

Overview of updates for SQL Server 2005 SP2

http://blogs.msdn.com/microsoft_dynamics_nav_sustained_engineering/archive/2008/01/11/available-updates-for-sql-servere-2005-sp2.aspx

3)  Some queries ran faster  on SQL2000 than they do on SQL2005

KB 942659 / SQL Hotfix bug number50001716:

FIX: The query performance is slower when you run the query in SQL Server 2005 than when you run the query in SQL Server 2000

This problem was experienced by a very few NAV customers after they upgraded from SQL2000 to SQL2005. The symptoms were: When changing a field filter (F7) on "Name" in a contact list, the response would take a couple of seconds. The same actions would give instant response on SQL2000. This problem did not cause drastic table scans, "only" 3-5.000 more reads than SQL 2000 would have done. But of course, in a multi user system this would cause a lot of unnecessary reads.

The correction was to implement a new trace flag 4119. Enabling this trace flag will set SQL2005's behaviour to that of SQL2000 for these specific queries. This trace flag is further documented in this post:
New trace flag in Update 4 for SQL 2005 SP2

This correction is included in cumulative update 4.