Use dm_exec_query_stats to find data for all statements in the cache: number of times the query has been executed, the longest query to execute.
Use these tow objects:
sys.dm_exec_query_stats
sys.dm_exec_sql_text
Run this query to find most frequently executed queries in SQL Server 2005
select* from sys.dm_exec_query_stats as across apply sys.dm_exec_sql_text(a.sql_handle) as b order by execution_count desct
Run DBCC FREEPROCCACHE to to clear the procedure cache before start monitoring.