Problem: I installed MOSS first, it worked well.  Then installed Performancepoint, the dasboard desinger worked well, but sharepoint site could not be loaded. from Event Viewer in Computer Management, I found the following error:

“Could not load file or assembly ‘System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35′ or one of its dependencies. The system cannot find the file.”

Reason:  deploying performancepoint on sharepoint site need ASP.Net AJAX extension installed but I only installed .Net 3.5.

Solution : downlaoding ASP.Net AJAX 1.0 and installing it