It could be one of countless things. I would start by checking the most expensive queries by CPU.

edit:

The whole story:

http://www.simple-talk.com/sql/performance/finding-the-causes-of-poor-performance-in-sql-server,-part-1/[/url]

http://www.simple-talk.com/sql/performance/finding-the-causes-of-poor-performance-in-sql-server,-part-2/[/url]