Microsoft's popular performance troubleshooting whitepaper has been updated for SQL Server 2008. You can download Troubleshooting Performance Problems in SQL Server 2008 at http://download.microsoft.com/download/D/B/D/DBDE7972-1EB9-470A-BA18-58849DB3EB3B/TShootPerfProbs2008.docx.

I've also added it to our whitepaper-links collection page at http://www.sqlskills.com/whitepapers.asp.