Slow queries on SQL Server
As you didn't provide us with any detailed information, may I suggest you watching the following videos from Brent Ozar (Very smart MVP). 


  http://www.toadworld.com/platforms/sql-server/w/wiki/10063.find-missing-indexes.aspx
  
  http://www.toadworld.com/platforms/sql-server/w/wiki/10062.find-indexes-not-in-use.aspx


These videos will give you a basic idea of how indexing works and give you scripts to run against your database and check for missing indexes and indexes that are not used.

I hope these links can help you, as they did for me.