Start by reading chapter 3 of http://www.simple-talk.com/books/sql-books/troubleshooting-sql-server-a-guide-for-the-accidental-dba/
Don't guess as to causes and fixes. Identify what queries are using a lot of CPU and tune those specifically. If you change settings at random, you'll never know what the cause of the problem was.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability