A new (version 2.0) management pack for ISA 2000/2004/2006 is avaiable for download. Microsoft’s comments are:
"The ISA Server Management Pack monitors ISA Server events and alerts for all ISA Server versions (2000, 2004 and 2006). With detailed alert information you can quickly identify and troubleshoot ISA Server issues, minimizing time-to-resolution when problems occur. Collect and analyze performance trends and metrics. Performance information allows you to manage bottlenecks, identify capacity requirements, and proactively manage your ISA Server configuration to resolve issues before problems occur"