If you are installing PerformancePoint Monitoring Server on SQL Server 2008 there are some prerequisite steps.  PerformancePoint Monitoring Server requires SQL Server 2005 client component software.  After SQL Server 2008 is installed and properly updated, then Cumulative update package 9 for SQL Server 2005 Service Pack 2 must be installed.  This CU9 is a hotfix and has to be requested at http://support.microsoft.com/kb/953752. 

After applying the SQL Server 2005 update, PerformancePoint 2007 can be installed but not configured until after PPS SP2 is applied.

Microsoft’s Technet has step by step instructions for these installations and updates at:

http://technet.microsoft.com/en-us/library/dd335966.aspx