What is good server performance monitoring software for Windows?

I've been experimenting with munin for monitoring around 8 Windows 2003 servers.

http://munin.projects.linpro.no/

Its a free linux-based system and the Windows agent works well & is easily extensible. Setup is simple if you have some minimal linux knowledge.

For performance monitor - start it on the server (Win+R and enter "perfmon"). Select "Performance Logs and Alerts" and expand. Select "Alerts". Select "Action" & then "New Alert". Give the alert a name, click "Add" to add a counter (there are hundres of counters, for example CPU %), then give it some limits.

Select the "Action" tab, and then decide what you want to do. You may need a third party program - for example Blat to send emails - but basiaclly any script can be run.