It's very rare that I tell people to blindy apply service packs or hotfixes (especially windows ones) without first analyzing where their performance problem lies, but if you are running SQL Server 2008 on Windows Server 2008 R2 (specifically this OS only) then you should absolutely ensure that you've installed the following fix:
976700 An application stops responding, experiences low performance, or experiences high privileged CPU usage if many large I/O operations are performed in Windows 7 or Windows Server 2008 R2
In this scenario, the key symptom which you normally ought to notice is that you are getting high kernel times, which can you see in the followiing perfmon counter:
Process(sqlservr)\% Privileged Time
We've seen a trend of this issue in recent months, so it's something to look to apply in your next maintenance window if you can.